Now this post is made without knowledge on the compatibility between the two (It isn't talked about in the cross mod compatibility in the information tab)
Now cerys uses plutonium for a bunch of purposes and adds a different way of obtaining it. I have no idea if these actually change based on whether or not are together or not.
(Also I can't figure out why the breeder reactor has not been functional for me in a long time which might be because of that, as in no techs around it)