Remington #10's fit my original Walker nipples just fine, and they also fit my 1860 Army, 1858 New Model Army, 3Rd Model Dragoon, etc. All made by Uberti.
I buy caps by the box of 1,000 so I'm not ready to change anything at this point. When the nipples wear-out, I might think about switching to #11's, but for right NOW, if it ain't broke, it don't need fixed!
As far as thumbs go, I've never seen a problem with correctly fitting caps being applied by thumb, so I will continue to do so. We have a full line of over a dozen shooters for a regular monthly match at a League that shoots year round. There has never been a problem in all of the years I've been a member......
Sometimes you don't need to just read about something to know better, you need to do it. Just one man's opinion, and thousands upon thousands of percussion caps later......
Dave