I’m not sure how those cams communicate or if they are supported yet but it sounds like several things might be assigned to the same rc input? The above link should get you on the right trick for configuring things correctly though. Hopefully that sorts out your last three points.
What gimbal specifically are you using and what is your objective with it? I don’t see tarot gimbals in the list on the ardupilot docs but they might fall under one of the other types as a close or something. Another options you might have is directly controlling the gimbal just as outputs if you don’t need the AP to be aware there is a gimbal. This will depend a lot on what you trying to do though. Its a bit janky but I have directly controller simpler gimbals that take sbus with the sbus servo out in ardupilot before SBus Servos — Copter documentation