Stud services between birds does present a few issues.
First there is the quarantine problem (Usually new adult birds are kept separate for more than a month to ensure that disease etc is not transmitted to either side)
Next comes the issue of hens laying. Stress can cause hens to stop laying for a period of time so in general you only want to move the boy. But introduction of a roo to a flock can be stress enough to stop hens from laying for a period of time
Next comes the issue of integration and mating in general a new roo may be challenged and not permitted to mate by the head hen. He may over mate a particular hen. Hens also technically have the capacity to reject/eject sperm from the storage locations they use to fertilize eggs.
I think bio security is the main concern though which prevents the easy integration issues.
