Is the 320 X5 Legion only milled for the Romeo optic or will the DPP or RMR also fit ?
Thanks in advance.

To use an RMR, you would need an adapter plate OR a re-mill by a gunsmith.

Springer Precision, C&H and Bobro come to mind as sources for plates. L&M Precision will modify your slide.

Thanks for the reply but maybe I wasn’t clear .
Is it cut for anything else but a Romeo ? Direct bolt on, no adapter plate ?

Might depend on when your slide was manufactured.
There have been some "rolling changes".
Take your plate off and see which cut you have :

I bought a Romeo1 Pro. My other choice was a DPP. Very happy with my purchase.