When Fenland skins was on the go, they were adamant that they wouldn't take skins from animals slaughtered so late. The wool comes off the skin in the processing, and blocks up the system, as well as coming out looking motheaten. We haven't had skins done for a while, but if we did it would be at 16 months (our usual age of slaughter for primitives), after shearing and allowing for an inch or two of even regrowth. 'Shearling' wouldn't be as delicious as lambs skins though.
Phone your chosen tanner to discuss directly with them, rather than going through all the work of salting, plus the cost of postage, only to find they won't do them.
With your crosses, they would likely be too fat after shearing. Best to shear them now before slaughter, but remember they have to regrow a certain amount before slaughter - they say it's for welfare reasons, but I suspect it's so the skins can be sent off to the fellmonger as usual.