That coralife (which Im looking at now on a freshwater tank) *should* fit a tank up to 24" wide with the mounting legs, which puts you in the realm of a bigger tank if you really like that light (otherwise the 130W 24" version is also pretty cheap there). From what I hear, 8mm glass is safe from a peacock, and a tank smaller than 60 gallons is very unlikely to have 8mm glass, so you might want to avoid one, or buy an acrylic tank. Plus, a peacock gets pretty big - I wouldnt put one in less than a 29 anyway (preferably much bigger). I wouldnt put anything in a 10 or 15gallon tank - 20 would be the way to go for a small mantis IMHO - N. Wennerae are small and very interesting when they dont feel threatened (and virtually invisible when they do).
I think you should think about a bigger tank, just in case you need me to spell it out. Because I'm so nice (and cannot sleep right now), lets run through what I would get if I wanted something that small:
24" Coralife 130W - $113.69 (hellolights)
AquaC Remora hang-on skimmer - $163.95 (marine depot w the maxijet). The prizm is junk compared to this. Skip the filter for now.
25 gallon (24x12x20) all-glass aquarium + stand (maybe $150 from *my* LFS). Less for the 20 (24x12x16).
50lbs aragonite sand (the tropical play sand). $5 from Home Despot.
Cup of live sand - free when begged from a local reefer.
The rest of your $500 goes on delivery and live rock. It's easier to add more live rock as you can afford it than to add a new tank when you can afford it.
Just my opinion - caveat lector.
Dave