What do you mean certain classes? There are three. Mage, Warrior, Rogue, and plenty of companions to supply.
Warriors; Sten, Alistair, Shale, Oghren, Dog.
Mages; Wynne, Morrigan.
Rogues; Zevran, Leliana.
Am I leaving someone out? Oh, Loghain, warrior I would think.
If you're asking if you can make the people that you recruit a certain class instead of what they're supposed to be, then you can't do that.