I've done a bit of research and weighed things up...
Kay, first off, before I decided to write this article, I read through MANY other articles regarding the subject. I wanted to see what other people were saying to get an idea of what to write. This article will mostly focus on the first film, but I have also included characters from the other two films.
Balto: Balto is obviously a Grey Wolf/husky mix. I say 'Grey Wolf' instead of 'Arctic Wolf' because the latter is merely a subspecies and all wolves are technically Grey Wolves.
Jenna: I think she's a Siberian Husky, as she's smaller than Alaskan Malamutes. The Alaskan Malamute is a large, stocky dog, and were used for hauling freight short distances.
Steele: I think Steele is a Siberian Husky because my research tells me that malamutes were stockier dogs designed to pull heavy weight short distances. Malamutes are also placed at the back of the team because of this, whereas lean and fast dogs are at the front. So yes, I now think he is a Siberian Husky.
(And none of that rubbish about Jenna and Steele's size differences. I'm fairly certain that not all dogs of one breed will be the same size. Differences ARE allowed. Plus....cartoon movie so shush!)
Nikki: Eh, people always say he's a fat husky/malamute. But looking at his short stature, obviously thick fur/build, teeny ears, round muzzle and more-often-than-not blue tongue, I wouldn't argue with him being a chow chow. Chow chows CAN be sled dogs, they just take more patience to train because they aren't SPECIFICALLY for racing but are related to huskies and other race dogs. While I'm here, I'd also like to say that the majority of background dogs from Balto 1 look similar to chows right down to the blue tongue. Of course some look taller, skinnier, bigger ears, pink tongued. But that doesn't mean they can't be malamutes, husky/chow crossbreeds, etc in there as well. Just generalizing on that bit.
Kaltag: Loads of people call him a Chinook nowadays, although that breed wasn't around in Alaska during the time of the serum run, so I'll rule that out. I'd like to say he is an Alaskan Husky, which is a classification rather than a breed. His build more closely resembles an AH (after a Chinook), with the longer legs, potentially floppy ears, and colouration/markings. Not to say that many background dogs don't have the same/similar markings.
Star: He is a hard dog to place if you're really looking. According to some people, the directors have confirmed that he was based off of the Klee Kai. However, that breed wasn't around until the 70s, so instead I think it's best to say he is a small Siberian Husky (which I believe is the closest relative to the Klee Kai).
Sylvie: I think she's an Afghan Hound with the build, style of fur, ears, nose, etc. However, I wouldn't disagree with some Borzoi blood in there due to her tail. But again, it's a cartoon and artistic license is allowed.
Dixie: People seem to adamantly say is a long-haired Chihuahua or Pomeranian. I would say she's neither. Chihuahuas are TINY and bug-eyed and very delicately built. While she is small and given a typical feminine dainty build, she doesn't really resemble a Chihuahua. Dixie's fur is far too different and looks a lot thicker. Pomeranians look closer to her, however they are still too small to be Dixie's breed. Dixie IS small, but she's isn't extremely tiny. Assuming that Jenna is roughly average size or just below, Dixie is only half the size of her. Put next to Rosie (I believe she's 7?), she still wouldn't be small enough to be either Chihuahua or Pomeranian. After looking through some breeds, I would say she is a Keeshound, a slightly larger relative of the Pomeranian.
Mort: He's clearly some kind of bulldog. I always want to say French Bulldog but he is stockier and has smaller ears. So I think the best bet for him would be an English Bulldog.
Doc: There's no other dog I can think of, or find, that looks like Doc. I think it's best to just call him a St Bernard crossbreed. Sorry.
Annoying terrier: He looks like a scotty, yet he's white like a westy. Oh wait! Scottish Terriers can be white too! There we are then.
Kirby: I never really knew what to place him as. I usually just called him a husky/malamute mix . But I have now decided that he has a closer resemblance to an Akita. He has a strong body and legs and isn't FAT but isn't skinny either. His face is rounded and fluffy, and his ears are proportioned the same as an Akita.
Ralph: I used to think Ralph and Kirby were brothers, but not anymore. I always thought that Ralph was more fluffy and big-boned than fat though. What kind of racedog would REALLY be fat after all? With his height I'd say he's probably a husky, but his build is stockier like malamute. However, with the bad drawing/animation of Balto 3, he could very well be an over-sized chow chow sorta of dog. I'm going to have to put him in the middle as husky/chow.
Dusty: Judging from her height, size, build, generalities and what not, I think Dusty is a Siberian Husky.
Mel; looks like an Airedale Terrier but much shorter, he is a Welsh Terrier.
Dipsy: I've always thought of her as a Basset Hound. She has long floppy ears, short legs, long body, wrinkles, somewhat droopy chops. Yeah, despite the weird short fluffy tail I would still call her a Basset Hound.
Mail team wheel dogs; likely malamutes due to size, build and position.
And that is about it. I don't think I've missed any (canine) characters. Hope the read was enjoyable and my points were valid.