Is Clickteam fusion 2.5 allowed to be used in this Game Jam?
It should be as it said that game engines such as UDK or unity3D are accepted. I also hope it is because I'll be using fusion for my entry.
That aside, I think that as long as we're all having fun and that we're being fair, any engine should be okay; as it is the main purpose of this jam.
I can't imagine why it wouldn't be.
Everything you can make games with is allowed ;)