Depends on the worm status of the paddock the lambs have been in so far. If they have been out grazing on a contaminated field (as in sheep on it in the last 12 months) then I wouldn't mix them. If their paddock is definitely clean, then there should be no problem.
Oh and also cocci, if these lambs are older they will have a higher level of immunity to cocci than your younger billy.... so maybe I wouldn't mix them.