So long as he's around 17-22 age wise it wouldn't be a problem. The Legion's main thing is you have to have internalized powers though. In the new52 the Nth metal can bond with Thanagarians and basically makes them similar to Wolverine but rather than claws and an unbreakable skeleton the Nth metal gives them armor, wings, and weapons. Or you could just save time and say Thanagarians have at this point developed natural wings and just say its a new character. Either way, throw something together, I like both your ideas so far. 

I like thinking outside of the box when it comes to DC's cosmic characters. There are so many aliens in Legion that would be awesome in GLC and vice versa. I'm working on the rest of my CS, but I'm hoping we've still got at least 5 players on board. 

~KL~