I was wondering if this is at all possible. I notice FlameAtronach and such are in the "Combat Style" list when editing an NPC. Is it possible to actually USE these? Like, make Sven float around like a Flame Atronach and do backflips and shoot fire and junk?

Yes, but you misunderstand what they do. Atronach floating and backflips are not a combat style, they are animations. The combat style controls their tactical decisions, for instance whether they fight aggressively or defensively, whether they prefer to tank or flank, and - if they can fly - how likely they are to strafe or land. If you had the tools, you could make Sven use Atronach animations, but that is an entirely different subject. It's also weird.