Do I need to invert the Sbus output from my FrSky receivers before connecting up to the CC3D Mainport ??
No. That is why you use only Mainport. It has a programmable inverter that is automatically used as needed.
Can I still select Oneshot125 with Sbus selected as RX Input ??
I haven't used a CC3D in a long time but I don't recall limitations related to SBus. I know there are more limitations with CC3D than Revo, for example the (Something)NoOneShot options on the Hardware page. I would use those as documentation. Also, there is not enough memory to run one of the available OSD options (MinimOsd, you must use MinOpOsd).
Are there any differences in setting up the CC3D Ie Receiver Channel End Points (Min and Max) etc
No. Nothing that I recall other than the known limitations of CC3D.