Anyone here paint cars as a hobby?

I enjoy it. I painted my first car in 1996. Since then I have painted 19 or 20 cars and trucks, plus some panel repaint jobs.
I almost always have materials left over after a job. Many times I have more paint than anything else. I'm curious though about compatibility between brands.
Do most reducers/thinners mix well within their type? In clearer terms, is it likely that an Omni single stage reducer would work with PPG paints? The paint store guys won't offer up this info unless I ask. I have found through some experimentation that cross breeding often causes no harm. I have used Omni reducers with Nason paints. I have used Sikkens catalyst with PPG.
Its not that I am some adventurous amateur chemist. Its just that with several cans of paint stuff with a few ounces here and there, it is a shame to just waste it. Sometimes I only need a few ounces for a spot repair or to paint a small part.