We are covering our two goats milk reared orphan lambs with a Hebridean tup, because the other ewes we have are pure Hebridean. The two ewe lambs are texel/mule crosses, so much larger than the Hebs, and so I don't foresee any trouble lambing them.
Previously I have seen mules and mule crosses on one farm tupped as ewe lambs to a Beulah. The sleeker mountain types are easy lambed, hardy enough to be up on their feet and drink quickly, so did really well, but they still have a decent conformation as the Beulah (or Kerry Hill etc) type is a large enough sheep.
It seems to me that ewe lambs may still almost remember being lambs, and seem to be quite maternal, where I have seen plenty lambed at 2 years old that get up and run away from the lambs.
I think if they are well grown, and the ram is a fine headed/shouldered type that will lamb easily and be fit, then covering ewe lambs is OK, but echoing others that best not to overfeed.