Would the same leather holsters be compatible with both the standard 3.7" barrel and the 3.1" (comp) barrel? I am assuming so, since the overall length is the same. The slide appears to be the same length with a different front sight placement on the comp barrel.

(And while you're here, feel free to suggest your favorite - OWB strong-side and ideally with adjustable tension.)

Yes, the overall slide length and the shape of the rail are what dictate fit with these.

Thus both a comp and a non-comp XMacro will share holsters since those are identical.

But a P365XL wouldn't, since even though the slide is the same length, the rail is a different shape.