IMO, unless ya get the MP's, more power heads are better than 1. A kinda of general rule, is 15-25% for a general FOWLR or softie tank, 25-35% for LPS, and 50-70% for an SPS. But that gets complicated with what certain PH and what corals, controllers, etc.
I have 3000 GPH, running in my 55g...