Found a P320 RX Compact today at Cabela's for $650. It's very tempting except I'd prefer a different optic than the Romeo 1 like the Trijicon RMR or the Holosun 507C. I am wondering if the RX slide can be modded to receive one of the other optics. I prefer the optic to attach directly to the gun with no adapter plate. Anybody know?

a romeo1 does not mimic an RMR in mounting if that is your question. so no you can't do what you want.
On the other hand getting a 320 milled for an RMR is pretty cheap.