The generic skinwalker gets a +2 bonus to one physical stat when transformed. Do they get to choose which one each time they transform like the bestial features, or is it fixed at creation?
As I read it they get to choose each time.
You mean the specific heritages? I agree those are fixed. Only the generic skinwalkers get to choose.
Yes, this is only for the generic (i.e. no specific animal heritage) skinwalker.
The reason I'm not sure is that for the bestial features it specifies that they choose one each time they transform, language which is lacking for the physical bonus. It could simply be that they neglected to be as explicit in that case, but it seems like it could also mean that a given skinwalker gets a bonus to one fixed physical stat when transformed.
This seems very lightly supported by the way skinwalkers of a specific bloodline have a fixed stat, but that could simply be for flavor reasons (plus that whole area is a mess. Why do ragebred and scalehearts get two physical bonuses? Why do three of them get the mental bonus when transformed instead of the physical?).