Is there a general rule of compensation for an outsider bringing companies together that results in a partnership?
While not general rules, here are some tips that might help: Call this a finder's fee and do a web search on finder's fees, and discussions of finders fees. It better not be very much. Most people who bring others together, resulting in a partnership, do it out of good will, networking, and assuming that a favor will be returned later. There isn't much economic value there. If you pay too much, the partnership fails because of it.