We've got 4 Shetlands and 2 Soays and more than likely some castle milk moorits soon (2 or 3).

Good, I knew resistance was pointless!
Lots of Shetland owners around me so that would be the easiest but is it OK to put him with first time Soays ? i know size difference isn't massive but is it a factor to consider?
At the sale there were some Beltex x Soay lambs; I have it on the best authority

that the tup was the Beltex. Very nice lambs, would make good breeders of fat lambs or would sell well as store lambs,
much nicer than the Texdale, Texwick or similar (Texel x from Swaledale, Herdwick or other hill breed ewe).
The couple who bought the CM ewe lambs (and then very kindly agreed to sell me the two I wanted) buy up all sorts of primitives and primitive crosses, including Soays, and put them all to a Texel x Beltex. Never have any problems, they said.
Not suggesting you use a Beltex or Texel, only meaning to demonstrate that primitives, including Soays, seem able to take a larger tup and produce sensibly sized lambs. Partly due to the wide pelvis of the more primitive types, no doubt.
I guess if it was practical to let the Soay girls have pure Soays for their first time, it would be a good plan. But I'm not sure I'd want to put Soay on the other breeds, (even though I know that size isn't everything

- but it
does matter

) so if it were me I would probably use the Shetland on them all. But I haven't had Soays so, like you, I'd want confirmation from people who've actually done it that it would be okay for their first time.