I would not let them go along with the adults unless they are 4 weeks old. So you need to find another way in between. I now have 11 one week old ones and I know how you feel about them getting crowded. I'm now waiting until the temps are back in the mid 70s so I can have them outdoors in the daytime. The would be the aviary, without the older ducks. That way they are protected from hawks etc. Then they will be back inside at night until they are old enough to move into the duck house.
Katharina
Katharina