Sage healer is really simple to heal as a multiboxer... I use to run a holy priest for my WoW 5-man. Basically I use have the same keyboard setup (minus the group heals which I don't have on my Sage yet). Also I second the benefit of not having all 4 of the same class in my group, as it gives you a second storyline/questline to follow... and weirdly enough I do enjoy the storylines/questlines.

My only concern with running Trooper Gunnery toons, is their potential heavy reliance on Full-Auto for thier DPS output?? I'd really rather use a toon that did not rely on a Channeled ability, though thanks to Riiker, I do have a good option to incorporate it into my setup now.