You can attach a compression valve directly to pex. The valves are the same as for copper, but you need the internal ferrule to strengthen it so the compression seal works. Or, if you use the copper stubouts, you can cut that off so you have the appropriate amount sticking out of the wall, then use a compression shutoff on that as if the whole thing was copper. Using the copper stubout and their brace for it would give the strongest connection. Since the shutoffs aren't really used much, I think either method would work well.
What you may not want to do is solder on a shutoff with the pex connection close...it might work, but I'd be somewhat worried about the heat from soldering it. If you use the copper stubouts with the brace, solder them to the brace, then make the pex connection to it. This may be overkill, but probably not a bad idea for the non-pro.