I've always mixed products since I started. And even more in the last ohh 10 years, since the majors have got so ridiculous on prices. And the majors value lines suck, I feel there are much better products from some smaller companies that are around the same price is the majors value lines. Can't remember any catastrophes in all the years mixing brands. Of course there are certain things you should never mix. One companys hardener usually doesn't work in a different companies paint, and many different mixing ratios out ther. You should always should use the correct hardeners for what you are using (there are companies that actually make hardeners to use in certain ppg, dupont ect paints, but I've never used them) About the only thing I've used a different brand hardener in is enamel at some jobs that had a universal hardener.
And certain lines you must use there reducer in, because there are more then just solvents in their reducer. Dupont chromabase, crossfire, reactive reducer. If its a line you could use another reducer in without it causing major problems, you must use a high enough grade of reducer. You should never use something like ppg omni reducer in ppg deltron line, although you could use Deltron reducer in omni.
But all in all, I find most companys undercoats and topcoats are pretty compatable with one another, so long as you mix the product you are using itself with the proper stuff. Of course, they would like you to use only their product, more sales for them. They do design products to work together, but you can mix lines. No blowups I remember mixing products yet that I remember anyways. You won't get any warranty, but unless you are some big shop with an account, probably wouldn't anyways.
Now if using certain things like rust convertors and other so called miracle products, well all bets are off.