Mine are doing something like that. I see feathers on the ground in the juvenile pen when mine hit about 10 weeks or so in age. Never been a long term issue. More like a little mini molt.
When to let them be together depends some on the behavior of the older hen. If they are big enough to not get killed in a small scuffle and she is not hostile I think you can let them be together after continuing your current set up in like a week or 2.
If there was a disease your older buff being so close to them would already be exposed or have exposed them so I would no longer worry about quarantine.
Mine are still separated since a few of my older hens are very very mean. My baby Brahma's are already the size of my 5 year old Delaware. I wont put them together because of the Wyandottes and a bad attitude.