That’s exactly the same as bringing weed seeds into a greenhouse along with cuttings of other plants, or bringing cockroaches into a building along with a box of books. The organisms are wild so long as they were not deliberately brought or maintained by a person.
So long as you aren’t deliberately feeding the guppies, treating them for disease, avoiding killing them while cleaning the tank or anything similar, they are wild. They just happen to be wild in an artificially maintained ecosystem, but that is even more true of a greenhouse weed or a household cockroach. The only difference is that you presumably aren’t taking steps (and failing) to kill your guppies, but that doesn’t make them any less wild.
Look at it like this - if everyone had a fish tank, and guppies became ubiquitous in all fish tanks, we would certainly consider them wild pests, just as we consider cockroaches or greenhouse weeds to be wild pests.