Gardenias aren't hardy here (except for one I just planted which is supposed to be hardy...smells delicious). You have to keep it indoors for the cold winter. I've never been lucky with them so maybe someone else can give you cultural directions. They get spider mites easily and need lots of humidity. Enjoy the flowers while they are there. Don't mess with transplanting while it's in bloom or the shock will cause buds to drop off.

I keep mine on my porch all summer, and bring it in when the temperatures start to drop in the fall. Two winters ago it made it OK in my living room. Last winter, I moved it to the landing, which has a south-facing window and is just outside the bathroom. It loved it, and is huge, but hasn't bloomed this year. It seems to love the humidity, though, and is always happier outdoors in the summer.

My sister has a massive one that's about five years old. She overwinters it in an enclosed patio that has no heat. She keeps it in a large box. When temperatures dip below 10 or so, she adds crumpled newspaper to the box and covers it with a sheet. Hers bloomed like crazy and is absolutely gorgeous.

I would separate them into separate pots if you can tell there are several plants, then put it outside for a while. I haven't had many mite issues, but then again on the porch there are enough predators to keep them at bay. If you're concerned, you can spray with a miticide or a horticultural oil, perhaps. Or, if you're feeling brave, you can wait, then do what I did this winter when I detected a few. I used those pre-moistened alcohol prep pads that you can buy in pharmacies for applying rubbing alcohol to an injection site, carefully swab both sides of the leaves and stems, and kill them that way. I don't know for certain if my wipes were pre-diluted with water (I think they're usually 70% isopropyl alcohol), all I know is I bought them to disinfect the phone during cold and flu season when the pharmacy around the corner was closing. None of the foliage burned or even discolored for me, and the mites were gone, one application was all it took.

Good luck.