An occasional guppy is fine. Guppies don
t have the same issues that goldfish pose (Mess, a fatty acid that saltwater animals have problems with, one that has led to blindness in some predators living on a diet of them, etc...).
I feed my P cilata guppies from time to time, although ghost shrimp are a better alternative, as well as thawed food, snails, etc...for your little monster.
Don't stress over a guppy or three, no big worries.
-Uriel
