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.
You can get 10,000k, 20,000k, 30,000k, I personally would recommend 20,000k as it has a nice balance of blue and white light, where 10,000k is too white, and 30,000k is VERY blue like deep reef colour.
I dont think its as simple as getting a MH flood light and changing the bulb. The ballast has to support the bulb type and the flood light type ballasts are normally rubbish when it comes to energy efficiency (high electric bills) and heat output. It can be done but you need to make sure you get a decent one that matches the bulbs you are using and even then you might not save in the long run when you consider the running costs.