What do you mean… compatible? A DJI model, a DSLR & Gimbal? Or a 2nd FPV forward camera?
The M600Pro has optional Gimbal Camera mounts… the X5 Mount and the Z30 Mount.
The X5 allows the older X3, Z3, X5, X5R & XT Thermal and the Z30 allows the Z30, XT2 & X4S to mount.
If you’re looking for a 2/3 sensor the Z3 is a nice small package with a nice zoom, for a MFT the X5R would be a good choice. These are DJI Cameras, so the direct interface with Flight App is clean.
If you’re wanting to mount a Sony, Red, Canon, Panasonic body you’ll need a RoninMX or 3rd Party Gimbal and additional components to fully control & view the camera in flight. All non-DJI payload options would greatly exceed your price bracket.