Lot of questions here, short answer is yes.
1. Yes this should fit any P320 FCU, but your california compliant barrel (LCI) will not be functional in this slide, as you'll have to omit the LCI part of it.
2. Our compact slides are of the 3.9" size
3. Romeo X will fit on our slides, but does need custom screws and a different shim - please email in to Service@greyghostprecision.com if you need more information!
For one of our compact slides this would definitely fit the bill (they can be used on M18's/Xcarry's/compacts) - do keep in mind that our slides are not compatible with loaded chamber indicators however. Depending on where you are at those are required by some states to remain legal!