A psychic Pokemon like gallade would be find but you already have a fighting type infernape so choose Bronzong or another psychic type Pokemon.
Depends what level they are. You can always improve (unless they are lv.100) you can teach new moves with TMs or HMs.
Yes, Infernape, Roserade, Floatzel, Drapion, Mismagius, and Garchomp make a good Pokemon team for Pearl.
make your floatzel learn icebeam by TM but level it up first because her garchomp has giga impact.
Get an alkazam or gyarados
Not bad but may I suggest training gabite to evolve to Garchomp??
It might be useful to have a water type since you don't have one yet. possibly buizel or floatzel?
Make sure you have plenty of revives and max potions. For Aaron, fire, flying, and rock moves are effective (Infernape, Togekiss, maybe Toxicroak) For Bertha, Water, Grass, and Ice moves are effective (Floatzel, Probopass) For Flint, water, ground, and rock moves are effective (Floatzel, Probopass) For Lucian, Bug, Ghost, and dark moves are effective (Giratina) For Cynthia: Spiritomb= Infernape, Floatzel Lucario= Infernape, Probopass, Togekiss Garchomp= Floatzel, anyone with dragon moves Milotic= Grass or thunder moves Roserade= Infernape, Togekiss, maybe Toxicroak Togekiss= Probopass, rock or thunder moves. I would recommend taking out toxicroak for maybe a luxray or roserade. anyone with grass or thunder moves.
If I have to say for myself, that is a good team. Teach your Infernape Flamethrower (can be found in Fugo Ironworks as a TM) and use it on the bug type. Use you Golbat and Staraptor to take out the Ground types, and teach waterfall to your Floatzel and beat the fire type. I think that Garchomp is effective against Phycic. That is all I will tell you, for you will have to beat the champion yourself. Make sure to stock up on Hyper Potions and Revives before you start.
arceus lv100 dialga lv100 palkia lv100 darkrai lv100 giratina lv100 deoxys lv100
honchkrow/staraptor (aaron), infernape/rapidash (aaron), leafeon/roserade/torterra (bertha), empoleon/floatzel (flint), honchkrow/spiritomb (lucian), glaceon/abomasnow (cynthia's garchomp)
a staraptor,torterra,garchomp,luxray,milotic,and a floatzel
Teach your Floatzel Ice Punch to use against Cynthia's Garchomp and Surf to use against her Lucario and Gastrodon.