I love your enthusiasm. Welcome to a whole new bag of crazy. I'm between Chicago and Milwaukee, so my winter is a touch colder than Grimlore's and yours, but not much colder than yours. I hate to be the one to tell you this, "bonsai won't teach you patience", maybe Tai Chi or Meditation will, but bonsai won't. However, there is a work around. MORE TREES. Load yourself up to the point you are always behind on tree care. At that point you will have enough to do on any given day that you can still set aside the ones that need to grow and leave those alone. I'm giving you permission, encouragement to get more trees! I need at least 50 to keep me from overworking to death my "good trees". Actually have over 100 sticks in pots so I am always behind on routine maintenance. A simple fix for patience, get more trees. If you collect your own, they don't have to cost money. Shopping for pots and assorted supplies is another task for the winter doldrums.
Tropicals. There are many that will work, though most benefit from a summer outdoors. Ficus is great for part shade to sun, most other tropicals need nearly full sun, south windows indoors, or a light garden with a long 18 hour day length. Longer days, say 18 hours, can make up for the lower than sunshine light intensity of a light set up. There are a fair number of threads on under lights gardening on this site. I have a Eugenia, aka tropical bush cherry, that is a great "indoor" bonsai for the south window or a light garden. Read up, there are hundreds of candidates for winter indoor tropicals and sub-tropicals. Even the culinary fig, Ficus carica, makes a good indoor bonsai - but it needs more light than the "houseplant type Ficus".
Names, I know its a pain, but common names are regional, this forum is international, over time try to get used to using the scientific names at least once in a post, so that readers will know which plant you are referring to. Yes, Sci names are a foreign language - technically not a language spoken by any living culture, made up of Latin & Greek root words, plus a good dose of words from many other languages. But with time, if you make the effort to try to refer at least once in a thread to the scientific names of your plants, you will get better advice, because there will be no confusion as to what species you are talking about.
On the tag for your "juniper" take a look, make sure it says juniper, because to my eye it looks like it is a Thuja or a Chamaecyparis. All 3 are good bonsai choices, but the Thuja and Chamaecyparis have different "rules" for pruning than a juniper. I could be wrong, pictures are 2 dimensional and limited, so check your tag. There is a Chamaecyparis "Golden Mops" that looks a bit like a golden juniper, especially in winter, and in planning future pruning techniques it is important to know which you have. Thuja and Chamaecyparis do not back bud on older wood, where Juniper back buds fairly reliably.
Our BNut Beloved Overlord is absolutely right, your job no. 1 is to learn the horticulture. Put the pruners away. I know, you can't stand it, but it will do you best.
About winter care - you said these came out of a cold greenhouse. That means they were likely somewhat protected. You are not wrong about being concerned about the temperate drop predicted from 50 & 60 F earlier this week to near Zero F coming tomorrow. Ideal winter storage keeps the trees below 40 F, do you have an unheated, or minimally heater garage or enclosed porch that will stay below 40 but 10 or so degrees above the outside temps. That is where I'd put the trees for the next couple days. Once the worst of the cold has past, they should go back into the yard, The spruces can go into full sun, as Grimlore mentioned. The "Mops" might be best in a spot out of the wind, and in shade for the rest of the winter. Once the pot is thawed and it has warmed a little, full sun for the 'mops' is needed.
While waiting for things to get growing, read up on bonsai potting media. You will need to pick your growing media. My recommendation is to seek out pumice if you can find it. One brand available at Saddle and Feed shops, or farm supply stores is a product call "Dry Stall". Be sure to read the labels, there are several similar looking trade names, but only one is pure pumice with no additives. It should be gray pellets, similar in appearance to Perlite, except quite a bit heavier. It is still lighter than some other commonly used media. Pumice is the one universal component found in most of the better commercial bonsai mixes. Or you can buy pre-mixed bonsai blends. This is more expensive and requires mail order. In Columbus OH there is a nursery that hosts the local bonsai club, it will carry potting mixes. In the Cleveland and Cincinnati area there are places too. Let us know if you are near any of those areas, and I'll give you addresses.
SO right now is the time to get potting mix, and some training trays. I like "Anderson Flats" they are 15 x 15 x 4 inch trays, with a open mesh bottom. They make good pots for the first step from nursery can on the way to a shallow bonsai pot. You can also make your own boxes, most make them out of wood, with mesh for sides and bottom. The open air mesh on the sides helps keep circling roots from forming. Read some of the threads about grow boxes and collendars, you trees are too big for collendars, but the principle is worth understanding.
So, there is plenty of stuff for you to do now. In less than 2 months you will be able to tackle repotting your trees.
By the way I really like your tallest spruce. That one in particular looks like it can make a very nice tree in time. Forget about "styles" and Styling for the next year or two. You will be needing to get the roots under control first. Remember with conifers, you can not work the top and the roots in the same year. No pruning the year you do a repotting. No repotting the year you do pruning of the foliage. One needs to be intact to support the other.
So keep us informed. Feel free to start a separate thread for each individual tree. That way you will get more specific suggestions.
Also, if you get a suggestion that doesn't make sense - don't follow it. Wait, read, and only when you understand the reason, and it makes sense for the tree in front of you "not just in theory" only then follow advice. The tree in front of you has not read the books, and may not be following what the internet says it should be doing. Remember to look at your tree and see if you can figure what it needs, rather than depend on "Internet Guru's" who have not seen your tree, or your yard. or know what your local climate is like. It is the tree in front of you that should inform your decisions.