There's no such thing as a "pygmy" waspfish...they're all small (3" - 4" range). Additionally, they have very small mouths, and they're bottom dwellers so they're pretty safe with other fish (they normally feed on small crustaceans). The "pygmy" thing must be salesmanship.
I'm guessing you've been looking at the redfins on DD? They're great little fish, but I do recommend weaning them in a QT before they hit the DT.