I've been digging around the site for a bit, but can't locate any place that provides a decent breakdown of guidelines for submitting custom pet overlays. Rules regarding image size, format, suggestions to avoid getting rejected, all that.

Is there anything else beyond this, be it an official FAQ or user-compiled thread?

If you mean the official overlays you can go to the submit overlay page and click on 'upload new'. It takes you to the rules/guidelines page for it. It won't take your csc until you actually upload it!

Here's a direct link!

, oh gosh I feel silly, I was so scared of that button believing it would take my cash that I never pressed it. Thank youuu!

I thought the exact same thing too actually but I know some sites which do pretty much the same thing so I eventually just clicked it XD You're welcome!