I do think you should consider changing moving mechanic to WASD as you said, because right now it might be a bit annoying to use the beam. And lowering skill count is fine, I wouldn't be probably able to use more than 6 active skills at once.
And yeah, combos might be interesting. Maybe they could appear at higher skill levels; like at 2nd or 3rd fireball level.
Also the fact that enemies can hit each other (I think only mages can do that right now) is interesting.