The metal bracket at the top of the arm was not very conducive for attaching the cameras I wanted, but it was a simple modification to make it work. The original bracket:

To modify, the left tab I bent around backwards, and the right tab I bent straight. You can see this in pictures below.

The pivot of the top arm has a friction gear system to resist the camera suddenly dropping if the release lever is pressed.(all the joints have these) This is great, but I was concerned about the cables getting caught in it. I made a protective shield with some packaging plastic, cut to fit, and folded. The shield:

Installed:

The original cable(in beige) I wanted to make available in case I want to work on the project more and motorize the focus and zoom, but I didn't want it hanging idol in the mean time. I bundled it carefully in the pivot area so it wouldn't get trapped or pinched:

To mount the different cameras, I picked up some thumb screws of appropriate thread to match the lens mount. For the still camera, a standard 1/2-20 knob bolt. For the video microscope lens, I couldn't find a knob bolt for 10-32, but could find some nice knurled nuts. Combined with a couple screws tightened in, it works great.

With matching holes drilled into the bracket, they mount each camera nicely. Here's each camera installed(after installing the rest of the plastic shell on the pivot):

Those pictures also clearly show how I bend the mounting frame.