I don't know a whole lot about the subject, but I can give simple answers to a few of your questions and I'll leave it to those who know better to elaborate (and correct).

I have never read any rules about rings that are particular to Highland attire, so you shouldn't have any problem wearing whatever rings you'd like so long as it's not too over the top.
Signet rings themselves are generally worn on the little finger, I've seen them worn on both the right and left hands and don't think there's a hard and fast rule as to which is appropriate.

A signet ring, by the strictest definition, is a ring that one would use to imprint one's seal in wax. Not all heraldic rings are engraved in reverse or intaglio, however, and I'm not sure those that aren't are still considered signet rings.

I don't believe a group ring would fall into this category.

As a signet ring bears an element of an heraldic achievement (usually the crest or shield), it does fall under the purview of the Lord Lyon and, when in Scotland, is subject to its laws.

Hopefully someone will come along to correct me if I have provided any misinformation.