Putting babies in same tank with adults (goldfish)?

[Hel]

When is it safe to put small goldfish into the same tank as large? The large range from about 3 inches (body only) to 5.5, while the babies are pretty tiny, half an inch to an inch (body only). I'm thinking not til the babies are at least bigger than the mouths of the big ones, but is there some other good metric?

(No, the babies aren't the babies of the large, but I figured the breeding forum would be where ppl would know about safely mixing fry with adults.)