Anything is allowed.

eyo not to, ummmm... be rude, but in my experiance, a better lined out page, listing rules, engines, and other things you can do, like teams or not, would attract more people and then you would have a bigger jam, most of the time when someone doesn't join a jam, it is the design of what they see.  they are looking to see what the rules are so they can see if their specific style of game disign is allowed.