They will survive lower temps than 55. I have kept them in my small garden pond for several years now. It freeze over the top but not solid.
I do not use koi because the pond is too small. I don't use goldfish either because I want amphibians to breed there too and gold will eat the eggs.
The rosies are easy to see in a dark pond and take care of mosquito larva. They breed and I do have lots of normal colored ones, a higher percentage than I started with, but after 8 years I still have plenty of rosy ones. I added a separate stream/pond and I only added the rose colored ones to it. I'm hoping they will not revert without a normal color to breed.