I just got a set of the Aquatinic reflectors in this past week. I've heard really good things about them, they are the same ones used in their fixtures and they are only 2" wide, so it's much easier to pack more of them in a smaller space (which is why I'm using them). I have always used Icecap reflectors in the past, and they are 2 1/2 inches wide, which is pretty good, but 2" is even better. The Aquatinic are made with Miro Silver, which is supposed to be about the most reflective material available for reflectors. I'll be mixing them with my current Icecap reflectors on my 220 as I set it up. They are a bit more expensive, but it will save me 2" total for the 4 over the Icecaps, which is well worth the few extra dollars to me.
As for how many over a 55, if it's an 18" tank, I'd probably do 4. It gives you more options for playing around with the color as you'd have more tubes to play with. And it would allow you to put an Actinic in and not worry about cutting down your PAR too much. I have 4 - 24" over a 29G that is 18" deep, and everything grows quite nicely in it.