2 6045's would likely be sufficient for a lower flow reef- colts, mushrooms, polyps. 3 of them would get you to where you could keep some simpler SPS, LPS and leathers nearer the pumps. My personal preference would be to use 2 6000 Streams and a multicontroller. With these pumps you would have no limits. Flow is really critical in reef tanks, not only for the health of the corals but for keeping the debris from accumulating and feeding algae which will grow much faster under the strong light.