Personally I never separate my goldfish when they're breeding. I just leave them in the pond let nature take its course and I usually end up with enough new fish to kep my stock up and still give some away. I would imagine that you could introduce them to the pond as soon as they are big enough not to be eaten by the other inhabitants (perhaps about 1" in length) Introduce them as you would any new fish "floating" them in a partialy filled plastic bag for a while so they dont suffer shock from temperature change and then let them loose, you might lose some but thats why fish lay so many eggs in the first place