I don't think there is any hard rule. The main suggestion I've seen: if you start having a bad hatch rate when the eggs come from a later generation, or your adult birds are smaller than previous generations, try adding new blood.About what generation would I need to bring in fresh birds?
Smaller birds would only be a problem if you are trying to keep up the size. If someone deliberately breeds the smallest birds they have, they could get some smaller-yet offspring without that being a worrying sign of inbreeding.
Poor hatch rate is definitely associated with inbreeding, but it is also also associated with incubator problems and various other causes. So do remember to check the other obvious causes if you get a poor hatch rate.
If he also has traits you want, then yes that would probably work well.Cookie does have a full brother (Tim) who's Cuckoo barred I believe. Could I pair him with golden commets?