For what it is worth, surely, if ewe mix (or tup mix for that matter) was so dangerous to feed to wethers or to the wrong sex, there would be a huge warning label on the sacks ?
My opinion is that the danger - such that it is - manifests itself over long term, rather than a coupld of weeks before slaughter so, I wouldn't make life any harder than it is and feed them all the same grub