I found that a lot of young people in the Balkans spoke English during my 2015 visit to the area, so I think you'll have some opportunities for substantive discussions with local residents as well as with other travelers you meet in hostels.
I see the night train to Sofia on the Deutsche Bahn website, but I don't think it carries any sleeping cars. That wouldn't work for me, but perhaps it's OK for you. You'll be changing trains in Plovdiv at 6 AM (if the train is on time, and I'm not sure you can count on that). Plovdiv is a lovely historic town, well worth a visit. I preferred it to Sofia, though there is stuff to see in Sofia as well. It seems a shame to blitz through Bulgaria, seeing only Sofia.
While in Belgrade I enjoyed a day-trip to Novi Sad. It has a picturesque old town with what I'd call a sort of hippie vibe.
If you haven't checked all the rail or bus links, you need to do that, to be sure your connections pass the viability test. For example, Rome2Rio thinks there's a Serbian Railways day train from Sofia to Belgrade and a Bulgarian Railways night train (33 hours!!), but the Deutsche Bahn website doesn't find a schedule for either one. I have a feeling that rail link may not exist. I assume you can cobble together some sort of bus routing, but you're traveling in mid-winter, and I don't know what the road conditions will be like. Transportation in the Balkans aside from Croatia and Slovenia tends to be quite slow. I figured I moved around at about 30 mph, no matter whether I was on a bus or a train. You have a lot of days that will be spent mostly getting to your next city.
I took buses from Sofia to Nis (southern Sebia) and from Nis to Podgorica, Montenegro, during September 2015. The leg westward from Nis was lovely, but I'm remembering mountains and wondering what that would be like in the winter.
Although you're traveling during the off-season, it would be prudent not to wait until the last minute to buy tickets for any cross-border buses you need to take. They run infrequently, and it's possible to run into a sell-out, especially around weekends and holidays. Pre-booking a few key long-distance trains might save you some money, but I'd do it only if the savings are substantial, because you'll lose flexibility with non-refundable tickets.
I don't know the best places to look for info on hostels, but booking.com typically has some apartments mixed in with its hotel listings, and hostels sometimes show up, too.
I loved Romania and think it's a shame you won't see it.
I'm a bit worried about the time of year you've chosen. I am definitely not a winter traveler.
