1) What kind of cable from gimbal to FC?
The flat one that comes packaged with the gimbal.
2) Does one typically power the GoPro from the system or with the internal battery only?
I use the internal battery on the GoPro.
3) Anything else to think of?
What guidance/GPS are you using? My Zenmuse H3-2D module interfaced directly to my Naza-M v2's PMU. It was all plug & play. Pretty simple really.
As far as mounting the gimbal I bought a Neewer FPV skid through Amazon. So far I'm happy with it. Lets you mount the camera/gimbal way forward to keep props and skid out of camera view. Also allows you to mount the battery on the bottom to the rear of the 550 to offset the weight of the camera equipment.
U.M.