Dude, these are a nuisance invasive species living unwanted in someone's pond in the "wild." Ecology 101: when you introduce a species that isn't native and it grows and breeds rapidly, native species can suffer. It's a matter of outcompeting certain niches in the food chain and leaving less food for the whole traditional native food chain. Sorry if you want to "save the plecos," but this isn't any different than destroying other invasive species like wild pigs. The OP is sharing with us some of the problems facing Florida and with her family in particular with invasive species, there's no way that pond could sustain that many large plecos without affecting native species long term IMO.