First rule of thumb is to quarantine your birds that are brought in from outside. Not having done so, you need to be extra aware of each and every bird's health. Catch anything early and be prepared to care for sick birds.
Second, I do not mix age groups of quail. I keep the ones hatched within a week of each other together and then start a new group if I hatch more the next week. I then sort out the ones I keep for breeding and growing in two different cages when they are older. Moving in new roo can sometimes work, if you only have that one new roo. Hens are hard on each other so I don't mix them if possible. This next year I plan on reducing my size of groups in breeding cages to one roo per two or three hens with a max of two roos and six hens per cage. Depending on cage size as I have a couple.