If I started with chimchar, what 5 other Pokemon should I catch for the..

Starrly, Shinx, Machop, Abra, and your choice.

Pick a grass type Pokemon, a water type, dialga, and some thing else just to fil the remaining 2 spots. Like a psychic, or lucario maybe..

A water type, grass type,flying type,ground type,electric type.This will help you around the game espicially for electric and flying Staraptor and Luxray.

Good Pokemon to start with would be

Shinx(electric)
Bidoof(normal but turns to water when he evolves)
Starly(absolutely positively awesome throughout the whole game)
Machop(fighting)
Budew(grass, evloves into roselia through friendship during daytime)

A very balanced team would consist of: Infernape, Roserade, Luxray, and Weavile. Good types to add to that team would be Dragon and Psychic.

With Chimchar, you'll need to catch Budew on Route 204 [or Roselia on Route 212], Shinx on Route 204, and Sneasel on Route 217.

A good dragon Pokemon is Garchomp [you'll need to catch a Gible in the Wayward Cave basement]. You can have Dialga in there, if you want, too.

A good psychic Pokemon is Alakazam. If you can't trade, you can keep Kadabra, too... [you'll need to catch an Abra wherever he can be found...] Azelf is a good one, too. She's one of the pixies. Bronzong isn't too shabby, he's actually pretty good. [You have to catch his pre-evolved form in Wayward Cave]. I'm a bit brain dead here, sorry I can't be much clearer.

Geez man why don't you just get one of us to play the whole game for you. Essentially all you have to do is balance types well. Youll need something electric, might I suggest catching shinx. Starly is a good early option for a flying type, though I prefer a murkrow. Youll need a water type, theres about a billion different types but if you are looking for an easy one to get early in the game, how about buizel.
Now it's also in your best interest to get something that can use the move earthquake, because it's that good, as well as something capable of using an ice move. Ill leave those ones to you. Its hard to pick 6 Pokemon and have them as your team because so many are so different, and you wont want your best 6 to have HM moves on them, because HM moves suck.

Well....Theres no specific Pokemon you have to use, but I usually use a fire,water, electric, flying, poison or grass, and a random Pokemon for fun. I usually use lucario as my random Pokemon since he rocks!

The way I beat the Elite Four with Chimchar as my starter was to get all my Pokemon to level 60 and have a starly which evolves into a saravia which evolves into staravia with the moves brave bird, fly, aerial ace, and defog...an onix with rock climb, rock throw, dragonbreath, and iron tail...bidoof which evolves into a bibarel with the moves waterfall, surf, cut, and hyper fang...an eevee, which I evolved into a leafeon by the mossy rock in eterna forest with bite, quick attack, tackle, and flash...and a geodude which evolves into a graveler whom you can only evolve to a golem when you have traded it...but my geodude has rock smash, dig, stealth rock, and strength. That way you have all the hm's in your party and there is a different variety of Pokemon you can choose from.

My bad...i meant to say a staravia that evolves into staraptor and my leafeon knows giga drain...hoped I helped out some.

Roserade, a poison grass poke
Luxray, an electric poke
Staraptor,a normal flying poke or Hurchkrow, a dark flying poke or Togekiss
Bastiondon, a rock steel poke or Rampardos, a rock poke
Palkia, water dragon poke or dialga, steel dragon poke
Sharpedo, a water dark poke or Milotic, a water poke if you don't choose palkia and Alakazam, psychic poke or Bronzor, a steel psychic poke if you don't choose dialga
Try to choose different poke type

A good champion team would be:

Infernape
Moves: Flare Blitz/Fire Blast, Rock Climb
Close Combat/Focus Blast, Sunny Day

Staraptor
Moves: Aerial Ace, Brave Bird,
Fly, Defog

Bronzong
Moves: Protect, Pyschic,
Gryro Ball, Strength

Roserade
Moves: Cut, Giga Drain,
Grass Knot, Toxic

Weavile
Moves: Night Slash, Ice Shard,
Aerial Ace, Rock Smash

Gastrodon
Moves: Surf, Waterfall,
Earthquake, Ice Beam

This is a good, mixed in-game team. No three Pokemon share a weakness, and I only gave them HMs for the fact that this is IN-GAME and HM slaves are a pain in the but to switch for all the time. Also, I personally don't like using legendaries (while they ARE fun to catch) because of how easy it makes the game. Hope this helps!

The best team for the elite four is a balanced type

Staraptor(brave bird,close combat,quick attack ,fly)
Floatzel(Crunch,Surf,Waterfall,blizzard)
Garchomp(Draco meteor,dragon pulse,giga impact,hyper beam)
Lucario(Focus blast,close combat,aura blast,flash cannon)
Legion(leaf blade,last resort,giga drain,quick attack)

I basically chose a different kind of approach to my team with Chimchar:

I chose Starly but gave up later on to get a Zubat then evolved it to a Crobat really early just during one of my battles in the 3rd gym and it's pretty well balanced same HP as staravia but 30 less ATK but it's still a solid 90 and a little bit more superior to it in DEF, SP. DEF, SP. ATK & 30 more SPD.

I then chose Psyduck as my water type cause I was too lazy to spend a LOT of time on a Feebas

Then as usual Luxray is a really good electric Pokemon if you have the right TMs

Gible saved me so I didnt have to use Graveler anymore plus it's Dragon type

Lastly for my grass type I chose Leafeon & Roserade, I mainly switch between them since both of them have decent HP, high SPD, but Leafeon has high ATK & DEF, while Roserade has high SP. ATK & SP. DEF, again boh of them would be so much better if you have the right TMs

Supposed to get Riolu or Ralts but don't need to since my psychic type is Psyduck & fighting is Chimchar
Overall it's up to your personal preference & this is really just an opinion from just another person her, but I'm really happy to be in this community