Nearly every time I battle, I make up my team on the spot, so I know what you mean there. I like to make them like this:
1) Physical Wall (Forretress come to mind)
2) Special Wall (Forretress? =O)
3) Physical Attacker (like that Dragonite)
4) Special Attacker (Gengar ^^)
5) Mixed Attacker/Defender (aka your Infernape)
6) Wild Card/Annoyer/UU Flagholder (Crobat fits here, I suppose)
I never thought of the Special Forretress, but it does make sense. And Heatran has good defences as well, and can be swapped in against anything trying to Flamethrower Forry.