Is this compatible with the 3.6" p320 x compact model/barrel? Also mine came with a romeo 1 pro red dot, will this slide accept this optic? — 2 Answers

Yes and No
Sig’s standard XCompact comes with a 3.6” slide a barrel. But the Velocity V1 P320C slide needs to be paired with a 3.9” barrel.
However the Velocity slide will fit properly on a XCompact grip module, the slide will just extend past the front end of the grip module 0.3”. I personally think it looks way cooler this way and is the same way Sig’s Spectre model of the XCompact comes from the factory. You just need to replace your 3.6” barrel with the 3.9” barrel and this slide will work perfectly on a XCompact frame. Google the XCompact Spectre and you will noticed the slide extends past the frame and does not sit flush.