Here are some cheats i know of that mmight help you all.

Play as Mecha Godzilla 2
Successfully complete adventure mode with all monsters, then complete adventure mode again with Mecha King Ghidorah.

Play as Mecha Godzilla 3
Successfully complete adventure mode as Orga on the hard difficulty setting.

Vortaak level
Successfully complete adventure mode with Mecha Godzilla 3 on the medium difficulty setting.

Gallery photos
Play through the game on hard difficulty setting. While fighting, various buildings that you destroy will have special Atari power-ups inside. Collect these to unlock a gallery picture.

Cheat mode
At the main menu, highlight "Versus" mode, then hold L + B + R (in that order), then release B, R, L (in that order) to display the cheat entry screen. Enter one of the following codes to activate the corresponding cheat function:

All cities and all monsters
Enter 863768 as a code. If you entered the code correctly, you will hear a sound.

Monster Island level
Enter 745749 as a code. If you entered the code correctly, you will hear a sound.

Mothership level
Enter 972094 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Mecha Godzilla.

Boxing ring level
Enter 440499 as a code. If you entered the code correctly, you will hear a sound.

All cities
Enter 107504 as a code. If you entered the code correctly, you will hear a sound.

All gallery items
Enter 962129 as a code. If you entered the code correctly, you will hear a sound.

All monsters except Orga
Enter 753079 as a code. If you entered the code correctly, you will hear a sound.

All monsters
Enter 209697 as a code. If you entered the code correctly, you will hear a sound.

Play as Godzilla 2000
Enter 637522 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Godzilla 90s version.

Play as Gigan
Enter 822777 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Aguirus.

Play as King Ghidorah
Enter 939376 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Megalon.

Play as Rodan
Enter 724284 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Gigan.

Play as Destoroyah
Enter 352117 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Godzilla 2000.

Play as Mecha King Ghidorah
Enter 504330 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with King Ghidrah.

Play as Mecha Godzilla
Enter 643861 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Destoroyah, Rodan and Mecha King Ghidrah.

Play as Orga
Enter 622600 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with all characters once, then complete adventure mode again on the medium difficulty setting with Godzilla 2000.

Black and white mode
Enter 860475 as a code. If you entered the code correctly, you will hear a sound.

Technicolor mode
Enter 394804 as a code. If you entered the code correctly, you will hear a sound.

Twelve continues in adventure mode
Enter 548319 as a code. If you entered the code correctly, you will hear a sound.

Add AI player to Melee (human versus AI)
Enter 154974 as a code. If you entered the code correctly, you will hear a sound.

Display game version number at option menu
Enter 097401 as a code. If you entered the code correctly, you will hear a sound.

Ending credits and play Godzilla March Theme
Enter 339223 as a code. If you entered the code correctly, you will hear a sound.

Player one is small
Enter 558277 as a code. If you entered the code correctly, you will hear a sound.

Even numbered players are small
Enter 600095 as a code. If you entered the code correctly, you will hear a sound.

Player two is small
Enter 689490 as a code. If you entered the code correctly, you will hear a sound.

Odd numbered players are small
Enter 853955 as a code. If you entered the code correctly, you will hear a sound.

Player three is small
Enter 203783 as a code. If you entered the code correctly, you will hear a sound.

Player four is small
Enter 495355 as a code. If you entered the code correctly, you will hear a sound.

All players are small
Enter 154974 as a code. If you entered the code correctly, you will hear a sound.

Player one takes no damage
Enter 152446 as a code. If you entered the code correctly, you will hear a sound.

Player two takes no damage
Enter 724689 as a code. If you entered the code correctly, you will hear a sound.

Player three takes no damage
Enter 367744 as a code. If you entered the code correctly, you will hear a sound.

Player four takes no damage
Enter 317320 as a code. If you entered the code correctly, you will hear a sound.

All players take no damage
Enter 569428 as a code. If you entered the code correctly, you will hear a sound.

Player one deals quadruple damage
Enter 940478 as a code. If you entered the code correctly, you will hear a sound.

Player two deals quadruple damage
Enter 930041 as a code. If you entered the code correctly, you will hear a sound.

Player three deals quadruple damage
Enter 537651 as a code. If you entered the code correctly, you will hear a sound.

Player four deals quadruple damage
Enter 889610 as a code. If you entered the code correctly, you will hear a sound.

All players deal quadruple damage
Enter 817683 as a code. If you entered the code correctly, you will hear a sound.

Military deals quadruple damage
Enter 970432 as a code. If you entered the code correctly, you will hear a sound.

Disable HUD
Enter 880460 as a code. If you entered the code correctly, you will hear a sound.

Players health regenerates
Enter 597378 as a code. If you entered the code correctly, you will hear a sound.

Throw all buildings and objects
Enter 248165 as a code. If you entered the code correctly, you will hear a sound.

Indestructible buildings
Enter 451129 as a code. If you entered the code correctly, you will hear a sound.

Hedorah (Smog Monster) in every game
Enter 288730 as a code. If you entered the code correctly, you will hear a sound.

No Hedorah (Smog Monster)
Enter 584408 as a code. If you entered the code correctly, you will hear a sound.

Player one invisible
Enter 659672 as a code. If you entered the code correctly, you will hear a sound.

Player two invisible
Enter 493946 as a code. If you entered the code correctly, you will hear a sound.

All monsters invisible
Enter 600225 as a code. If you entered the code correctly, you will hear a sound.

Energy does not recharge, but energy weapons do more damage
Enter 690242 as a code. If you entered the code correctly, you will hear a sound.

Toggle military
Enter 728629 as a code. If you entered the code correctly, you will hear a sound.

Player indicators always visible
Enter 860068 as a code. If you entered the code correctly, you will hear a sound.

Constant full energy for player one
Enter 778393 as a code. If you entered the code correctly, you will hear a sound.

Constant full energy for player two
Enter 881557 as a code. If you entered the code correctly, you will hear a sound.

Constant full energy for player three
Enter 597558 as a code. If you entered the code correctly, you will hear a sound.

Constant full energy for pplayer four
Enter 218967 as a code. If you entered the code correctly, you will hear a sound.

No health power-ups
Enter 221086 as a code. If you entered the code correctly, you will hear a sound.

No Mothra power-ups
Enter 491040 as a code. If you entered the code correctly, you will hear a sound.

No energy power-ups
Enter 803358 as a code. If you entered the code correctly, you will hear a sound.

No rage power-ups
Enter 666500 as a code. If you entered the code correctly, you will hear a sound.

No freeze tanks
Enter 223501 as a code. If you entered the code correctly, you will hear a sound.

No power-ups
Enter 229497 as a code. If you entered the code correctly, you will hear a sound.

No power-ups except Energy
Enter 553945 as a code. If you entered the code correctly, you will hear a sound.

No power-ups except Rage
Enter 660398 as a code. If you entered the code correctly, you will hear a sound.

No power-ups except Health
Enter 270426 as a code. If you entered the code correctly, you will hear a sound.

Player one always in Rage
Enter 159120 as a code. If you entered the code correctly, you will hear a sound.

Player two always in Rage
Enter 491089 as a code. If you entered the code correctly, you will hear a sound.

Player three always in Rage
Enter 450514 as a code. If you entered the code correctly, you will hear a sound.

Player four always in Rage
Enter 702905 as a code. If you entered the code correctly, you will hear a sound.

Well, thats every single cheat I know for Godzilla Destroy all Monsters Melee, enjoy!