Well the general rule of thumb is to keep it within the franchise's "relem of possibilty". If it is crossover, keep your fic within all the franchise you have chosen. However, if it is an alternative universe fic, all you just have to do is to keep the apperances,names and possibly their personality, everything else you can make up.
I have a fandom discussion board on my website's forum if you are intrested.
04-Apr-2006