You should be good to add the IMU off the back, but you just want to be careful you don’t knock it off in transit. And I think the magnetic compass was disabled on the current software release. I think it’s Inertial only right now.
For the GoPro, my friend Scott added one like this to the bottom. He uses it for making up wreck mosaics, but it can certainly be turned forwards. I think slinging it to the bottom is the best way as it adds a sort of “keel” to the ROV to balance it out.
I was thinking about the thruster placement as well and I went into my copy of The ROV Manual to check to see if there was anything on the subject and there is not for how high or low thrusters should be placed on the frame. Center of Gravity (CG) is not the only factor in ROVs, there is also a Center of Buoyancy (CB), which I am taking into consideration.
I have a thought going that the closer to the CB the thrusters are, the more stable it will be. Is it true, I don’t know, I haven’t done the math, but I keep looking at the larger ROVs and all their horizontal thrusters are placed pretty high on the frame and very close to the floatation. The most I can do is try it!