Can Rex fuck the centaur?
Yes, but the scene is in the version 0.26, and the following information is a spoiler, so you're aware.
The sex scene is a interaction between Rex and: Centaur, Anastasia, Rin, and Cloy.
However, Cloy and our Centaur are in their human form.
What about in centaur form, when is that planned?
I don't have a date
Yes, it will definitely happen! ;)