I would start with what you got place the sps up high then is no or little growth add the 2 strip fixutre at 54w bulbs with what you got will give you 5.8w per gallon thats way plenty if you add a single 54w strip your sitting at 4.9w per gallon which would be pleny without heat issues. I am running 2 (4blub) fixtures for a total of 432w or 4.8w per gallon no heat issues even under a canopy and have sps/lps and softies. and all are doing good.