Goldfish owner here... don't nobody be talking **** about my Ryukin. Lol, jk, but seriously, if you're just going to feed some goldfish to your FBTs, they won't have long enough to be "dirty". The reason they're so dirty is that when you have an aquarium, they produce more ammonia than average per fish. Thus, you need more filters/substrate which the bacteria that transforms Ammonia into Nitrite lives in. The other reason they're "dirty" is because they tear up live plants. I highly recommend java moss.

But, back to your question, Mollies would be good, and yes, small goldfish won't actually be "dirty". And keep in mind, that, goldfish, being direct predecessors of wild carp, are very bony fish, I.E. high in calcium.

Just make sure to find some that are small enough for your FBTs!