Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
The ruleset is quite simple:
-no legendaries, mythical and battle beast
-You are only allowed to use a single Pokemon
ANYTHING else is allowed that you can think of, as long as you can do it in the usual Pokemon Games (refering to moves a Pokemon can't learn).
I had a discussion about this...