I have done just what you are planning to do with my iPad Pro 12.9 both last year, with the beta for iOS 11, and now with the beta for iOS 12. I do use an iPhone X, and I have not experienced anything out of the ordinary at all as far as iCloud and photo sharing goes. Actually right now I’m having no problems at all and am planning on installing Public Beta 6.
I would leave you the standard caution that this is beta, and therefore things may change.