First thing to consider is a healing bot is just that, a bot. If you are going to rely on it to lvl 70 and beyond, it's going to kill you more than a few times

Tank bots however are a bit more manageable. #bot group attack will send it off to a target, and the bot mostly seems to try to get agro on anything that hits it, so healing one is a bit safer.
As for combo's.. It depends on what you intend to play.
Pallies are notrious for agro stealing.. so work good with rog clr shm wiz?
Rog bots will jump behind mobs whenever they can, so playing a tank class with a rog bot is good dps.
I started with a rng /shmbot combo but got sick of the mob getting dotted to death and the shm being out of mana to heal me. Switched to a shm box and a clr bot and survived after that a little better.
If you can run a 2nd char as a box... do. Then just work out what you want a bot to do for you.