That is one limitation that I'm going to try and stick with, but I don't know if its a rule. I really need two action buttons for my idea but have come up with a way to possibly do it with one to match the Atari 2600 joystick.
I guess you probably wouldn't get judged if you added an extra button, but maybe not a whole keyboard of keys. My answer is just an opinion. So don't take it as an official answer.