Xiaomi Redmi Note 6 works great. From what I've read the issues come with phones that are not fully compatible with AR technology. It's perfectly possible that while you can't pre-register you might be able to download the game once it's out, though. AR seems to be an added bonus to the game, but not entirely necessary, so perhaps they're limiting the devices so it's easier to manage the beta release. They might make it available for everyone once the proper version is out.
As for the specs, I think they were mentioned in a post in the blog here. They seem to be the same as the ones for PókemonGO.
Hope that helps!