I'd put one XP2 on each, then the AC300 on the 75 and one of the Penguins on the 55. Load up the upper media basket of the XP with bioballs or ceramic pieces for max biological filtration. I know you can put more bioballs in the AC
, not sure about the penguins. As for the PHs, if you felt you needed one to stir up a dead corner you could do it, but I'm not sure they'd be necessary.