I think callsigns are the least of your worries. Seriously, a crossover fan fiction, from four distinctly different universes? How could you possibly make this plot feasible or coherent without making quantum leaps in logic?
Furthermore, if you really want to be a good writer, you shouldn't write stories about other characters: make your own story, with its own players and its own universe.
Honestly, this is just not a very good idea.