tail

I mistakenly tried mixing and matching a new trv and a rad tail that were supplied separately. But I don't think they're a great match. I should have used the original matching trv/tail, but the tail olive has now been tightened once so..how do you get tail olives off to replace them?

What height would you generally have the 1.5mm tail sticking out of the plaster? What length tail should be ok? What are the considerations.. Thank you, Dain