LPS would be fine in a 550 with the standard lighting maybe even some of the easier to keep SPS Montipora Digitata and plating Monti beyond that you will struggle imo you could add a clip on Halide but I suspect the evaporation would be hideous.
Maybe a ATI Sunpower 6 x 24W and you could keep most SPS if that stats are very stable stick with the standard lighting and go LPS/Softees there are some good examples out there.