We use temporary tags at birth for both Soay and Hebrideans. We use the tiny 'turkey wing tags' which are like a 1cm square metal padlock. They are put in by hand. Then at 4 months when the lambs leave the dam we put in one of their big tags - the second we put in when they are older or when they leave the holding. Soay in particular are always sticking their heads through fences or getting into headlocks with their chums, which tends to pull out the larger eartags if put in when they are still tiny.
For our final tags we use Bubblegum, which look a bit wide but are nice and soft and do very well.
There are some slaughter tags which have a very slim pin which some people use for small-eared sheep. I worry that they will not last for the life of a Soay as they are intended to be short-term.