I want to run 1" trunk and 1/2" branches to fixtures, and would prefer to use PEX A mainly for its flexibility. Unfortunately my local stores only carry PEX A in 1/2 and 3/4 sizes. Would it be okay to mix 1" PEX B branching to 1/2" PEX A? I was also not sure if I would go with crimps or expansion, but based on what materials are available to me, I'm thinking it would be best to go the route of 1" PEX B with crimps versus downsizing to 3/4 inch pex A.
As long as the fittings at the transition points are compatible with both types of PEX, go for it
Plumbing codes generally permit the materials they allow to be mixed freely provided suitable transitions are provided where materials change. So, you can simply use a fitting that's compatible with both PEX-a and PEX-b at the transition points.