If you go with a blue spot DO NOT get the pacific variety. It has a beyond dismal record in captivity. There is a hardy atlantic species thats similar but does much much better in captivity. I would reccomend whoever you get it from you make sure it's eating first. There are a number of other rays that do very well in captivity but I think most are borderline temperate species - wetwebmedia has a lot of information on elasmobranchs - also check out the 'aggressive' fish forum on Reefcentral - specifically ask Mike Dandenau about rays , he seems to have kept a lot of tropical atlantic species and you might even be able to find something locally that might do ok in your tank.
Also rays have fairly delicate skin on their underbelly much like sharks - get some fine grain sand and I'd limit decorations to provide lots of swimming room.
For this kind of tank I'd say whatever skimmer you think will work - double it! If you are interested there is a good book on sharks and rays in the aquarium by Scott Michael - other than some of the reccomended tank sizes being a bit small it's an excellent reference.
Good luck! When I got back into fish tanks a few years back I originally wanted to do freshwater stingrays then found out they are illegal in texas :(
30 Gallon reef, 220 gallon South American Cichlid tank.