Toshiro Yuki
The issue is likely that many eras have different levels of technology. Some Gundams even have limited energy sources while others are nigh infinite. Some have advanced combat computers like Veda or the Zero System available. Some can beam spam and others can't. Balance in such a Crossover RP tends to be a huge problem.
Those are the biggest issues that would occur, and it would be difficult to decide how to deal with such unique designs. I think it might be best to create a story and then ask for unit suggestions, and from those create their weapons, capabilities, names, etc, and this would allow for some things that aren't necessarily gundam to be added.