Depends what you're gonna do with them.

Elemental shamans are fun in PvP, but a few BM hunters wouldn't do bad either. Especially with a priest following them. BM hunters could even work in arena, most specifically 2v2/3v3 bracket, moreso then elemental shamans. Elemental shamans shine in 5v5 though.

For PvE you could use a tank ofcourse, so that's where the warrior can come in. I dont see much prospect of a warrior being effective in pvp while multiboxing tho. :P

Personally I'd go feral druid instead of warrior tho. It's more managable for a multiboxer. :>