JO, don't what your zone 6a encompasses during the winter. I had lived in Santa Fe, NM and that's also zone 6 but I was someplace at 7000 in altitude. I read somewhere that if cold hardy plants are outside and sort of established by late summer then they could take the cold winter outside. I would think the semps in your pots would be able to do that (they are alpine plants). The tender succulents are another story and since they are mixed in with the alpine plants then I would consider the garage near a window? (Is garage heated or not?) Sometimes in NM, I would bring certain pots out like yours (mixed) and place in winter filtered sun for a treat.
Experiment: Would you consider taking out the tenders and put in another pot while leaving the rest (if established enough) to winter outside? One less pot(s) to lug into house? Then you can put the tenders back into the semp pot in spring...
