Google isn't a company that restricts their services in such a coupled manner. I fully expect it to be supported on most devices where it's possible to support it.
In my opinion, restrictive would be keeping Stadia to only Google specific hardware. Making it available for all Android devices feels pretty good to me.
I would actually suspect more of the opposite. Their biggest hurdle, at least initially, is fighting the perception that the service just won't be performant enough to be a good experience. I would not be surprised if they start with a limited set of high-end devices (Apple ones included), just so people don't blame device limitations on the service instead.
