Welcome to the Guild.
Check out the Guide if you'd like some info about how the site is set up. Using the Ask a Comrade thread and/or private messaging a Comrade is a good option if you feel the need to ask some specific questions about things related to the site.
As for your question, no, there are no limits other than the no adult content rule (keep everything PG-13 and you'll be fine). You can create whatever kind of RP you like, be it a standard play by post or a full on D&D or other tabletop game. The only issue with the latter is that there is no built in dice function for the forum, so you'll have to play those kinds of games off-site. I've seen both rolz.org and roll20.net used for D&D games started on this forum, so check them out if you're interested in that. Also, Mahz, the site owner, said a little while back that he plans to implement a Tabletop Games subforum and integrate a dice system for the forum to go with it. No news on when that will happen, but it's a possibility for the future.