RE th CAE issue - I would have a word with your vets up there, just I don't know if this is as much an issue on Shetland as it is on the Mainland where goats come and go on a much largfer scale. Your vet will tell you if a) you need your boy tested and b) if them you can insist on any female to be tested.
Also I personally wouldn't board females, rather send the prospective goat owner a smelly rag from your billy to help her owner decide when the female is in season and then she will have to come on the day or the next. Most goats stay in season for 48 hours, especially in the main time Nov - Jan. However if boarding is the only option then you would need a separate pen for the incoming girl and they probably shouldn't be running all together outside. There is always quite a significant biosecurity risk with other gotas coming on to stay with yours. When I have boarded my females before I have taken a good amount of feed with me so she can have what she is used to in the "goat hotel" too. On returning I have wormed, treated with spot-on for lice and other creepy crawlies that may have hitched a ride. The goat would then stay in in her own pen for a few days to allow any worm eggs/lice to come out/fall off. (I have found that even if the nannies go out for just the mating they inevitably come back with lice...
The male owners I have taken my females to either always have a small pen/secure area where the mating takes place, with only one boy being present!