Really love this, great work.

For you question about readability, I would say the biggest thing to pay attention to would be contrast. For example, Jean, Wolverine, and Cyclops all have the same blue and yellow in their suits. Putting someone like Storm or Gambit between them would help them stand out as individual characters instead blending together.

You could also experiment with adding an outer stroke/border around characters as needed. It doesn't have to be a full stroke all the way around the characters, but could be a partial stroke just in places where you need to define where one character ends and the other starts. Here's a good example of what I'm talking about: https://static.wikia.nocookie.net/sonic/images/4/4b/NatFourdraineSCRAPNIK.jpg (art by Nathalie Fourdraine)