OK, I just noted the reply from Nessus, so I'll take it as affirmative, NEVER mix paints. In fact, I should toss out the others just so I never get tempted (lots of nice pearls and metallics there to cause no end of trouble.
Needn't toss them out. Just make sure you keep them segregated in your workflow by cleaning. Or, since you have multiple brushes now, assign them to separate brushes so you can use them concurrently without having to clean.