The yellow-edged variety looks like 'Golden Oakland', which is a hybrid that isn't a type of Japanese Holly. (There are a couple rare forms of variegated Japanese Holly on the market, but their leaves would resemble those of the 'Sky Pencil' in being small and spineless.) Plant breeders online (possibly the one that introduced the variety recently) state it will mature around 15 to 20 feet high and 12 to 15 feet wide, if that helps you plan where it should be planted. Hollies tolerate pruning fairly well, but in general, plants should not be cut back drastically as a means to manage their size, so try to let it get as close to its eventual size as possible with minimal pruning. 'Sky Pencil' can reach roughly 8 to 10 feet high and 2 feet wide.

Planting time doesn't need to be based on air temperatures, as soil remains warmer well into early winter, allowing for root growth. Granted, the top inch or two might freeze during a cold snap, but as long as you can dig, you can plant. The more risky element with planting this late is the plants being evergreen, since foliage that remains in winter still loses water vapor just as it does in summer, so roots need to be able to rehydrate the plant by having access to unfrozen water. If new plants aren't rooted-in by the time the ground freezes regularly, they will be at greater risk of dehydration unless they can be monitored and watered as needed (during a thaw), and provided that they aren't frost-heaved out of the ground, which happens when the freezing soil pushes the root ball out of the planting hole, exposing it to the elements. A good 3- to 4-inch layer of insulating mulch over the root zone can help to avoid this after planting.

It will be riskier to keep the plants above-ground all winter, as root balls not insulated by surrounding soil are going to experience greater extremes of freeze-thaw cycles and will dry out faster, requiring more regular checks for watering. Options for delaying planting until spring, if you cannot plant now due to lack of a dedicated spot to put them or the soil being too difficult to work, include:

• Heeling them in: this technique "plants" the plant without removing the container, so it has the benefit of being surrounded by soil while also being easy to move to its final planting location in spring. Nurseries regularly do this with potted trees and some shrubs as a means to overwinter things without being able to plant them. Since the plants are still only growing in potting soil and have no easy access to the surrounding soil with this method, they will need regular checking for watering needs. Potting mix dries out much faster than soil.
• Heeling them in above-ground: instead of putting the pot into the ground, build-up "ground" around the pot, into a sort of hill or berm. Soil or mulch could be used for this, and as with the above, the pots themselves will need to be monitored for watering needs during temps. above freezing. (Just as during summer, feel the soil a couple inches deep or so. If somewhat dry at that depth, watering is probably needed. If damp, watering can probably wait.)
• Keeping them cool but not frozen: while there is a chance that the plants will fare okay if not protected and just watered as needed, such as being placed up against a south-facing exterior wall, keeping them chilled but above freezing would be ideal. They need to stay cool for winter dormancy, so they can't come inside, and nurseries that overwinter extra inventory in greenhouses or "hoop houses" vent away as much built-up heat as they can, as the goal is to keep the plants as cold as possible while avoiding freezing. If the sunroom is unheated (or minimally heated) where it stays in the 30s or 40s, that storage option may work well. Again, check them for water regularly, though it's possible they'd only need watering about once every week or two, depending on how much the room warms up on mild, sunny days.

If you opt to not plant them into their final location until spring, you don't need to wait for "last frost" in spring the way guidance recommends for tender plants; as soon as the ground is workable (not frozen and not soggy), they can be planted. If we have a mild winter, hollies planted in the ground now will probably keep growing roots most of the season.

Miri

You're welcome! Sounds like a good plan.