I'm a plumber, but I've never used CPVC except to make a minor repair once on an existing system. I'm just curious - why the rubber washer? I know the PEX x male adaptor has a tapered thread (pipe thread), and I would assume the CPVC x female adaptor does also(?) If so, there's no need for the rubber washer - just some teflon tape or pipe dope. Does the washer come with the CPVC fitting, or were you planning to add it?
Edited to add:
I just googled it, and saw that the washer is integral to that fitting, so you shouldn't use it with that PEX adaptor. I'd use a CPVC x male
adaptor (all plastic):
and a 1/2" PEX x female