I've got 18 skinned and salted. Gonna tan them myself. In some I have some nicks where I have a small slit that I did while skinning, Usuall on the belly or chest. I want to sew them up after pickling and tanning. The question is what thread to use? I figure a waxed thread, but in the past for sewing leather I used a .8mm thick thread. Being that these hides are so thin/light, what size thread should I look for, or what if available to sew up the slits/holes? What type of stitch would you use for that?

Use Braided fishing line. Works great and strong. I am not sure on what the stitch is called. But basically make your fist knot, then just loop over each section. Sometimes its better to make the hole a bit bigger or different to sew up correctly. I know people use dental floss as well.