Paint and solvent compatibility: mixing brand X paint with brand Z reducer...?

[Kern Dog]

Fairly simple question, but maybe a complicated answer?
I paint cars as a hobby. Sometimes I help friends and paint their cars, other times I paint a car that I intend to sell. I pften have materials left over. In fact, i have several cans of various chemicals, many with just a few ounces remaining.
I am curious about using solvents/reducers/catalysts of differing brands than the paint that I have. Example: I have Omni single stage that I have mixed with some Omni base coat. I mixed them together to get more coverage on a beater pickup since I only had about a quart of each. It covered fine and had a semi gloss sheen, which was okay for the application.
Other times i have had some PPG single stage black but no PPG reducer. I used some Omni reducer and it turned out just fine. With a few minor success stories, I may have answered my own question, but I'd like your input.

 

[wannadrag]

Omni is made by PPG,Demension by Sherwin Williams and Nason and Dupont.Buddy of mine mixes and sells paint and says dont mix and match but if you keep the stuff made buy the same company together it will work 99 percent of the time.

 

[Kern Dog]

I have had good luck so far. I have mixed same brand base coats with single stage.
I wish that I could remember the specifics, but at one time a paint store guy said....We're out of stock of THAT reducer. You can substitute Acetone though. THAT was good news. Acetone was about $10 per gallon compared to $35 for the "correct" stuff.