I probably have like 20 different tanks now, but I mostly just use my mini nautilus' and protank 3's with all the protank 2 stuff as a backup. So I'll usually have 2-4 tanks on the go and when swapping flavors I try to keep them at least in the same ballpark as the previous one. Peanut butter can easily become a custard tank and different fruits switch easier than to something not in the fruit category.
But hey... one of my favorite mixes is BC Blast mixed accidentally with Komodo by Unique Vapor Innovations which is a vanilla pastry flavor, I didn't smell before I refilled and turns out I have the best tasting fruit pastry mix I've ever tried. It's like a heavenly pie.