you might find this useful thenIf I didn't have these remarkable critters, I'd move heaven and earth to get me some. They are somewhat rare. I checked Privett's breed list and Buttercups are not on it. These tiny things must have been a little experiment or something. They have zero problem fitting in with the larger chickens. They are fierce and independent. But they do let me cuddle them occasionally.
https://www.rbst.org.uk/sicilian-buttercup
clearly some of this is not what you have found (e.g. not hardy, egg size), and given they are so rare, it may be that individual characteristics have been given here as breed characteristics; perhaps you would report back what differences you would highlight?