Side dishes with salmon at a dinner party in January...

You are getting some nice options to try here.
A vivid green vegetable has a lot of eye and taste appeal with salmon. One option would be to do spinach with some lemon zest and pickled ginger. An Asian style rice would be nice with that.
Spinach or another vivid green would be nice with slivers of garlic and either lemon zest or preserved lemon.
More traditional would be a green veg and a Hollandaise to dress the fish and the vegetable.
Or, given the guest's various diets maybe just baby potatoes dressed with tarragon and butter.
I don't think root/winter vegetables necessarily compliment salmon. But you might think about leeks that are braised or poached with stock and/or cream.

salmon is such a beautiful protein so i try to pair it with a dark or brilliant green such as broccoli or asparagus. both flavors stand up to the flavor of salmon. i also like rice flavored with garlic or onion. polenta provides a beautiful color contrast but can be overpowered. salmon is so special for us in salmon country that we tend to really "do it up" and not just with some sort of plain potato or noodles. mmmm. i wish we had some right now.

I really like rice pilaf with slivers of preserved lemons (lemon zest would work) and this broccoli dish from Ina. It really is perfect. I was thinking of trying it with broccolini, but my store has been out, so I haven't, but I think it would be delicious.

http://www.recipetineats.com/magic-broccoli/

Creamed spinach, roasted potatoes, mushrooms and onions, rice pilaf, green beans with slivered, toasted almonds.

Here's a hotline thread that may give you some good ideas:
https://food52.com/hotline/12159-any-suggestions-for-quick-and-easy-side-dishes-that-go-well-with-salmon

A salad of mixed lettuces , topped with chunks of roasted sweet potato or butternut squash, garnished by pomegranate arils or dried cranberries and your favorite vinaigrette.
Rice salad from silver palate cookbook or a classic Indian pilaf.
For dessert, aged parmesan served with vin Santo or port.

Forbidden rice , roasted vegetables , lentils, cauliflower mash ....