With plenty of filtration and regular water changes you could keep 50 fry in each of the 20G tanks until thay reach just over 1 inch. The chances are that when the fry are released from the cave they may already be 1/2 inch long..!!
As to when you can put the on the 60G tank it will all depend on what else you have in there. BN fry make tasty snacks for lots of fish... Angels, Kribs large mollies other cats etc.... If nothing in the tank that is likely to snack on them then there is nothing to stop you putting them in at any time (ie sraight away).
I have found that they will grow quicker in a bigger tank.