We are making our 8th January trip in 18 days!! We will be there from the 21st-29th!!!!
Since you are coming from canada.. and Im assuming its colder there than it is in Pittsburgh, You will welcome ANY weather as long as its warmer than where you are from!
January is a hard month to predict. We have had every range of weather from 28 degrees as the low in 2003 to upper 80's as the high in 2002. Most trips Id say is 50ish at night, upper 60's/low 70's for the high. I usually pack all capri pants, and short sleeve shirts. I pack 1 pair of jeans, and wear one on the plane. The kids pack half shorts, half capris ( for the girls) I take a light jacket and we usally have a heavier one on. I also take a few sweatshirts, our rain ponchos, and some of those cheap stretch gloves. Then, the night before we leave, I look to see if there are any major weather changes. If its calling for really low lows for FLorida, then I add long sleeve shirts and more jeans. If its calling for warmer temps, I add more shorts for the kids. MOST trips.. shorts and capris have worked for the day. Some trips we have to change into sweatshirts/jeans/jackets when the sun goes down. Some trips we stay in shorts/capris and it stays warm all evening. You just really never know. Im hoping that the east coast temps stay milder and hopefully that will keep Florida warmer
As for swimming.. the pools are heated. We dont usually take much time out from the parks to swim, but my kids, and the families we travel with, love to swim at night when we get home from the parks. The kids swim no matter then temps outside. THey were in the pool the year it was like 40 at night. Just bring towels and something warm to cover up in when you make the mad dash back to the room!

Id say, chances are, you will have SOME day out of the week where you will be able to swim if you want.
You are picking a great time to go crowd wise. It will such low crowds that you will get to do quite a bit. Test Track, Buzz Lightyear and Splash Mountain will all be closed for refurbishment. Splash is open I think on the 21st or 22nd, then closes until the 27th or 28th.. so there is the possiblitly of riding it. SPectro is being shown on Saturday the 21st, Monday the 23rd, Thursday the 26th ( this is also a EMH day for Magic Kingdom, so Id avoid this day.. its going to be more crowded ).
Where are you staying? There are 18 of us staying at POP! I cant wait!