Mike
I take it that you know about all the differnt types of thk? If not you want the 4 way equal loading like HSR or SHS, the SSR and SR can only be used horizontally, but if cheap enougth they would be ok for the x axis.

I under stand what you mean about the single rail but once you see a 25mm HSR you will realise just how strong they are,especially given the small cutters you are using. That said, I would probably use a pair of 15mm rails for the x axis and then single 25mm one's for the y and z

If you mount the x rails and blocks upside down then the block ('s) is (are) always directly below the cutter giving max strengh at all times