I noticed my battery ground cable (going from terminal to frame on side of battery) is pretty thin, probably about 12ga, and has been replaced because it has yellow crimp-on rings on it. I assume this should have around a 8ga cable like the positive, correct?

I don't know about the size, but have you checked the on line info at buykawasaki.com? I think thats it. they have complete parts diagram for the bikes, also have you checked out gYpSy'S web site. she talks a good deal about the battery cables/connections as well as a multitude of other VN7xx issues. Good luck.

I noticed my battery ground cable (going from terminal to frame on side of battery) is pretty thin, probably about 12ga, and has been replaced because it has yellow crimp-on rings on it. I assume this should have around a 8ga cable like the positive, correct?

My main ground is around 8g I think. It's def a thicker than 12g.
When you fix it, you might run another ground from that frame point to another good ground using 10 or 12g wire....

I've got some left over 8ga from a previous amplifier install, so I'll use some of that. (I think using the 1/0 cable I've got might be a little overkill...:) ) I just need to pick up some terminal rings.