Feeder goldfish just means "cheap". They are to feed to other fish, like Oscars and Piranhas and stuff. They're just like other goldfish, just not fancy and they're pretty hardy.... same kind you get at contests at the fair.
Anyway, a regular size water trough most people buy for a few horses is not near big enough to stay full or be an ecosystem, you'd have to feed them, and the horses would be drinking waste water. I don't think it's a good idea.
I prefer smaller water troughs filled daily to large ones that turn green and gunky anyhow.
We don't have a float yet. Ours is a 100 gallon and we have to fill it every two / three days. When I re filled it, it was clean. Just a new toy for the boys here.