Mateus and Balmung are the main RP servers on the Crystal data center. I'm sure you can find RP on other servers/worlds but it's on those two in abundance. I just checked on both and both are currently congested, meaning you can't make new characters on those worlds. However you are able to visit those worlds as long as you have a character in the server those worlds belong to, being Crystal. I know that's a bit of information but I hope that helps, if you need clarification on anything let me know