If your real question is, can chicks carry diseases, the answer is yes they can.
If you are looking at the pros and cons of mixing versus isolating the newcomers, I'll try to discuss. I think there are advantages and disadvantages both ways. And some may depend on how the chicks were kept at the breeder's. Were they isolated or with the flock and exposed to more possibilities? Even if they were isolated since hatch, they still could carry certain diseases picked up by common equipment or air born diseases, but the risk is less.
If you keep them separate, there are certain diseases they might have that could show themselves in the quarantine period. Or they might have a disease that they are immune to and will not show symptoms but could give to other birds. These will not show themselves unless you introduce them to new birds. So, if you keep them isolated by group, you won't know if they are carriers but are immune themselves. But you will know if they have a disease that has not yet manifested itself and you will know which group is carrying that disease.
If you combine the new groups, you take the chance of losing them all, but you protect your home flock from any diseases that one group has but is immune to plus any that has not manuifested itself. You may not be sure which group is the carrier, however. I think combining and quarantining the two new arriving groups together will do a better job of protecting your home flock.
The other aspect of this is that your home flock may be infected with something they are immune to but will kill off any new bird you introduce to them. This is probably not as big a concern as the other way around, but just something to be aware of.
Convoluted logic perhaps, but a good mental workout on one cup of coffee. I think I'll go get another.
