JDM66,
i really would wait until you have the S800 in hands and have the frame setup. At the moment (maybe you have the info) we have no clue what dimensions the landing gear has. Were and how things can be mounted to the frame. It looks like
they are mounting the zenmuse pretty much in front of the frame and balancing it again with the lipo. If this is possible with the AV200 or CS is the question. If not than you have to mount it on the center unter the plate and find a new spot for the lipo. There are to many unknowns that it would be risky to buy a gimbal right now and on good luck hope you will make it fit, inorder to keep your copter balanced the landinggear out of sight and it might even be to small to take a CS gimbal 2-axis under it between the landinggear etc.
Boris