I think it depends for me. I can handle them being in any situation so long as their characterization is right on from canon. I could never see Enjolras as an Ultra or as a brunette in fanfic. One goes against his beliefs entirely, the other a symbolic representation of why he was written in the first place. But it's the characterization that I'm always so picky about and won't read the fic if that can't be backed by their canon facets.