I don't think Grunt was referring to their enviro-suits when he said quarians were not as soft as humans. If he was, then presumably he was referring also to armored asari, humans, and salarians? And I, at least, would say the armor we've seen for them so far looks thicker and tougher than quarian suits. Further, he leads into this discussion by describing how far he had to stick his finger into a human to break the spine - presumably he didn't mean he could poke a finger through armor, but just through human/asari/salarian flesh. Then turians would require some effort with a blade, while quarians are somewhere in between. If he was referring to the armored forms of all of these creatures, presumably they'd all be pretty tough.
I suppose it could go either way, but I like to think they have some kind of armor.
Regarding luminous eyes - I think they're quite possible. There are deep-sea fishes with luminous eyes (not reflective, like those of cats, but genuinely luminous).
Regarding skin tone - I think pale isn't unreasonable from a design perspective, at least. Regarding natural selection, I think the OP was referring to the human body's tendency to build up pigment in response to sunlight - hence, a human, always clothed, would likely be lighter than one who spent their life in sunlight. That said, it also seems unlikely that there would be an evolutionary pressure to be pale, excepting the possibility that skin pigments in quarians may, like in humans, have a role in radiation protection, and that in an environment where there is no radiation to avoid, those quarians that do not waste energy making pigments would be fitter than those that did. Look, for instance, at cave creatures, which tend to be pale pretty much across the board. That said, that's presumably a quite weak evolutionary pressure, and a mere 300 years is unlikely to change the species that much.
I see no reason to assume they are mammals. They have breasts, which, if we presume they are for lactation, carries some mammal traits along (presumably if they nurse they are altricial - their young are born relatively helpless and are nursed for a long period). Other than that, though, I see no reason to assume they have any other mammalian qualities.
Stepping away from the science, if I were to develop a design for them (and I'm trying), I would try to make them something like desert reptomammals, with thickened skin (so not plates like a turian so much as rhinoceros-like hide). From a design perspective I'd also want to make them look as much like geth as possible.