I've been stuck in the same boat, I was given a set of both Honda and Mazda MX3 seats. My red capri needed seats.
It's really just a major PITA. I tried swapping sliders, no good. I tried welding new nuts into the mount points, and my welder was giving me fits and the metal is really thin. I tried several options but gave up as I was on a tight on time, ended up grabbing seats from my GTXR2 temporarily.
You can remove the stock Capri sliders, make some brackets, and mount the bare seats on top of them.
I haven't seen the focus sliders, but you could perhaps use them & make brackets to fit the chassis. As long as they're well built I don't see a problem with that. Hiding them under carpet or painting to match can help reduce their visual impact
Good luck, post pictures!