Some tips for choosing your starting class and not doubling up with a recruitable NPC.
Late late game, aka The White March Expansion, has the following 3 companions:
Monk
Rogue
Barbarian
These classes can be gotten in act 2, which depending on how much you focus on the main story can be reached relatively quickly, or if you take your time, pretty deep in:
Cipher
Ranger
Paladin
Druid
You get the following classes in act 1, very early, so if you play as one of these classes you're pretty much guaranteed to double up for the first chunk of the game:
Fighter
Wizard
Chanter
Priest
It's also worth noting that for a small gold fee you can roll your own companions with the classes/skills you want if you choose to fill some gaps in your party or if you prefer rolling your whole party Icewind Dale style.