I wonder if it would make sense to put the goldfish fry in some netting / put up a net tank divider until they're a bit bigger and can fend for themselves. I think the only concern would be that the plecos eat them if they're not fast enough, not in the sense that they would hunt them purposefully, but if the goldfish fry end up in the way somehow...? Though I guess the fry would have to be on their way out if they couldn't get away from a plec... I'm no expert on this but I have a feeling that you should be ok, but if you're concerned you can always set up some kind of net or divider.