I have 2 mini rex bunnies, one has sore hocks. They have always been active bunnies and always had a large run. When they were young, their run was on concrete (I didn't know about sore hocks then.) As a result, one of them got sore hocks. I feel terrible for not knowing about it and because of that, she's had to live with sore hocks for the rest of her life. She's now 6 years old, and she's under close veterinary monitoring to make sure she's not in pain.
In my opinion, rexes should only be kept on soft flooring, which is consistent throughout the whole accommodation. My girls are kept in a hutch with run attached by run around pipe. Throughout the whole setup, there's soft hay. I use a layer of megazorb litter and a layer of hay in their hutch. In the run, there is foam matting and then a thick layer of hay. There is no surface that isn't soft. Hay is good because it allows the nails to sink in, reducing pressure to the hock.
As your rexes haven't got sore hocks yet, you could probably use foam or rubber matting. However I would keep a very close eye on the rabbits hocks to check they aren't becoming sore. If they do, then use hay for flooring. It's best to prevent sore hocks than treat it.
If you need any more info about this type of flooring, just pm me.
