i've seen these breeding units that you can put in a tank to keep the fry in to protect them from the other tank inhabitants...they hang on the side of the tank...would these work?

breeding trap or net eh ?

I am not sure, because I haven't had any goldfish babies myself... but there are two types.... the netting and the plastic cube.

The breeding units themselves are nice because you can keep them in the cycled main tank and the unit protects them from excess filtration. The only problem is what to do as the fry get older.
The netting would probably be better for fry until they get big enough because they are generally larger and allow no chance for fry to escape into the main tank. The downsides are if you use it a lot.... you will have to become an expert seamstress to keep the netting in reasonable shape.
The plastic cube is actually what I use in case my adult fish get sick or stressed. The cube helped my fantail through an ornament experience, where he got cut up pretty badly and was floating belly up. It kept him away from the others nipping at him and from the heavy current. In three days he was back to his old self. The problem with the plastic cube is that there is little water exchange between the inside and the outside, so you need to 'flush' it every once and a while and the fry may be able to escape from the holes.

Overall, it would be a good temporary solution for holding fry until you got a main tank set up for them.

yeah the et ones do look like they would be better and im sure one could be built wit the right materials for little cost