I have a bare optic-ready slide for my P365XL. The cutout for the optic is in the forward position, just behind the ejection port. The footprint is in the Shield RMS/SMS pattern, as is the Romeo Zero.
The Holosun 507K-X2 optic uses this same footprint, but I would rather use the Holosun 507C-X2 which has the larger Trijicon RMR footprint. Has anyone here mounted a 507C to a P365? If so, did you use an adapter plate and which one? Did it require taller fixed sights to see them through the optic in a back-up roll?

You will need an adapter plate because I'm pretty sure the RMR style is too wide to fit on the slide. However, I don't know of anyone making plates for the P365.

The above is correct. An easier way to get a larger window that is direct milled is to mount the full sized EPS. It’s got the same mounting footprint as the carry but the larger window.