If the Shetlands have lambed before as you say, they should be just as capable as a commercial ewe even with a bigger ram, due to their special confirmation which is ideally suited to easy lambing.
Of course there is a slightly higher risk of complications when you use a bigger ram, but no more than if the ewe and ram were big.
NB this doesnt apply to all primitives, Shetlands are a bit unusual, tho it may be fine with some others.