These are some of your hang-ups? Not the crazy powers, all the aliens, that one guy who can transform into a hulking green rage beast, or the talking raccoon?
Not all planets have breathable air, I guess, since Star-Lord had to use his mask most of the time in the very opening sequence of Guardians of the Galaxy, right? But, hey, space-movie-science, what do I know? Also, he's part-alien. Or, maybe he only uses it for show. I'm really guessing, here.
Also, that the various characters can all understand each other? One of the
GotG most (in)famous easter eggs is that they all have some kind of translator implant - it was shown in the scanners at the Nova Corps facility, when they got arrested. It seems kind of easy to surmise that this technology would be easily replicable.