Never tried parrot feather so can't tell you anything about that. Hyacinth is a good one, as is water lettuce. Anacharis and hornwort will grow free-floating but don't float on the surface like hyacinth, but are both very fast growers. The plecos could probably go in the pond for the summer, Depending on your location I would be worried more about the water getting too hot. Then there's the question of, what happens to them when the summer is over?