ChaosControl2007

Remember the Digi-Elves you recused on the NumeNume River? If you saved all 10 of them, then they are your tickets to obtaining alternate Digivolutions. Fair warning though: each new form starts at level 1, so there's a lot of work to be done.

The sidequest names are:
Barbarian Cave
Undead Yard
Sealed Shrine
Captive Dungeon
Trader Hole
Dungeon of Doom
Item Road
Cursed Dungeon

Seeing how you started with Agumon, here's what I've discovered you get and where:

Alternate Rookies (go to Digi-Lab at appropriate level)
Dorumon -> Agumon at level 16
Guilmon -> Veemon at level 16
Veemon -> Dorumon at level 16
Normal: Undead Yard (destroy all 279 enemies)
WarGreymon X
Hard : Undead Yard (destroy all 302 enemies)
ClavisAngemon
Hard : Sealed Shrine (escape with 10+ minutes remaining)
WereGarurumon X
V.Hard: Sealed Shrine (offer a Defense ROM B and escape with 10+ remaining)
Susanomon
Hard : Captive Dungeon (defeat Scorpiomon with 1 HP remaining)
Alphamon
V.Hard: Captive Dungeon (defeat Scorpiomon with 1 HP remaining)
Imperialdramon Fighter Mode
Hard : Dungeon of Doom (return from the dungeon with Sword Cannon r)
MetalGarurumon X
V.Hard: Dungeon of Doom (return from the dungeon with Blaster Gun r)
Callantmon Crimson Mode
Normal: Item Road (defeat ShogunGekomon)
HerculesKabuterimon
Hard : Item Road (defeat ShogunGekomon)
Beelzemon Blast Mode
Hard : Cursed Dungeon (defeat LordKnightmon and return)
BlackWarGreymon X
V.Hard: Cursed Dungeon (defeat 2-3Lord Knightmon and return)
Imperialdramon Paladin Mode