It would probably be frowned upon to post in casual and advanced; one or the other's the way it goes typically. Both have different connotations at any rate.
Doing a regular category + nation/general happens semi-frequently. You can get away with that no trouble.
Something like Arena generally only goes into Arena.
You probably wouldn't get in trouble for doing two regular categories (I haven't heard of anyone who has), but doing free/casual/advanced and/or throwing the RP into additional forums probably wouldn't go well.
Doing a regular category + nation/general happens semi-frequently. You can get away with that no trouble.
Something like Arena generally only goes into Arena.
You probably wouldn't get in trouble for doing two regular categories (I haven't heard of anyone who has), but doing free/casual/advanced and/or throwing the RP into additional forums probably wouldn't go well.