In zone 7, you are safe to transplant as late in fall as the soil is workable. Soils in zone 7 don't freeze to any significant extent - couple of inches maybe and then only if a prolonged cold spell, usually occurring much later in the season. Roots will remain quite active until soil temps drop below 40F and hardy geraniums are very forgiving and root easily. Mulch if you feel the need. In zones 7 and above, one does not really need to be scared of winter gardening. I am a bit milder to be sure, but I plant and transplant all winter as long as the weather is the slightest bit agreeable.

Jenn, do I understand correctly it's been outdoors (so does not need hardening off from being in a completely protected environment) and is doing fine now? I would think you would be safe in planting it. If I had a containerized hydrangea that had been outdoors and not in the house or in a greenhouse, I would plant it here in my Z8 if only my ground were not too saturated to dig. With all the precipitation we've had, I couldn't possibly take a shovel to my beds right now :) On the odd chance something very unusual were to show up in your forecast (not likely, but it does happen), you could drop an old sheet, tablecloth over it temporarily but I doubt very much if that kind of insurance would be necessary. Light overnight frost shouldn't give it any pause at all.

Creosote kills/prevents new plants. Established plants or vigorous plants are no problem. Steep in hot water for a few minutes, soak overnight, and plant directly in desired spot to avoid transplant shock. Creosotes are notorious transplant difficult divas, so I just grew mine from seed. Creosote plants require about one cup of water every month for the first 3 years. Then you can stop watering them. As long as the desert you live in gets some humidity caused dew that settles on the branches and drips to the root of the plant, your creosotes will be fine. This is the undisputed champion of the drought survivors. Keep the water away, and growth with flowers will be yours. Young bushes like to have a rock placed next to them to the South, for shade. The roots will seek lower temperatures under the rock. Here is a link that might be useful:

I never got the Zones myself, personally. Gardening in west central Georgia (Zone 7A) is nothing like eastern Pennsylvania (Zone 7A). Somehow I doubt you'll see too many weeks with sustained 15 degree nighttime temperatures in west central Georgia, nor weeks of blanketed snow (which you'll still see here). Yet the zones are identical simply due to an occasional extreme in temperature in that part of Georgia and our extreme being closer to 0 and tightly bordering Zone 6? Then trace along Zone 7 and look at all the places it encompasses. Identical weather through the year, identical enough to mean the same gardening and agricultural habits and methods? Yeah, I think not. :-) When given a zone in the lawn forum, I just ask for more information. It's practically useless--it says nothing about actual sustained winter temperatures, rainfall, snowfall, and so on. Frankly, I don't rely on it much in terms of plant survival, either, and look up reports on how things actually do. Zone 3, for the US, is at least small enough that I'd believe things might be relatively similar in some cases.