If you interfere with a baseball player that is trying to make a catch in the stands, is that interference?

If the spectator reaches across the wall into the field of play, it is ruled fan interference. If the player reaches across the wall into the stands, there is no interference called. Even if interference is not called, it is possible that you may be ejected for poor sportsmanship or some such nonsense - policies for ejecting fans do vary by ballpark. If you are simply trying to catch the ball fouled into the stands, I don't see how you could ever be tossed out.