Flying Dragon (Poncirus trifoliata) rootstock should also help with hardiness as well as control size - but I'd encourage you to get out the wheeler and try and get them into the greenhouse this winter. You've got a nice collection going, and it would be a shame to lose it. Watch out, though for branches with distinctive trifoliate leaves (they'll originate below the graft). They're from the root, not the scion, and should be pruned back to concentrate the plant's energy on the grafted stock.
I just bought a few and left it outside for the summer. They will be brought in under a growing light in the winter :)
