buy whatever valve you want. then,,if it is a threaded valve, buy a threaded 1/2'' x 1/2'' pex adapter if it is sweat conection get a 1/2'' fitting x 1/2'' pex i caution you....I would not run anything to your shower but 3/4'' size if i was using pex volume, you want volume in a big shower, if you run 1/2 it will be like a old man peeing...dribble dribble

Like Frodo said use adapters to connect PEX to valves. The main valve has 2 mounting holes to secure valve with 2 screws but you don't always have something behind valve to screw it to. rigid copper pipe allows a more rigid installation. You can secure the pipe. The diverter valve doesn't have mounting holes . If you use rigid pipe you can secure the valve with the pipe. I suggest pipe every thing from the mix valve to the diverter valve and on to heads and hose adapters to use all rigid copper. Then PEX hot and cold to main valve.

update: I spoke to my plumbing supply person who said they don't make multi-functions valves that are compatible with PEX. She thinks the only way to do what I want to do in my shower is with copper pipe. thoughts?