The original team definitely did, but I wouldn't put it past Billy to try and put together more versions of the team. Like his age, he seems to be stuck in time, using the same methods over and over again. Adding slight changes, but never able to to make any actual improvements to his plans.
Yeah, the thing is though I don't think Khyber would add them to it since, compared to most aliens in the Ben 10 franchise, Xenomorphs really aren't much of a threat.
I don't think that can happen. Though shown to be capable of learning, Xenomorphs seem to act much more on animal instinct. Also, while it may not pertain to 5YL, Derrick Wyatt confirmed they wouldn't be accepted into the Omnitrix. They could go in the Nemetrix though.
Anur Millgan is an anagram of Gillman, an alternate name for the Creature from the Black Lagoon which the native species is based on. Unfortunately, Anur G'rrnay comes from the word gurney which doesn't really help us figure out what the natives are like. I always figured they were based on skeletons, but I also have a vague memory of reading somewhere they're supposed to be giants.