These models are reskinned versions of four characters from Half Life 2. From left to right: Dr. Isaac Kleiner, Father Grigori, The "G Man" (actual name unknown, even though he first appeared in the original Half Life over a decade ago), and Odessa Cubbige. They've been modified to resemble Team Fortress 2 characters and you can get them here.
You'll notice that they're clothed in a rather blue motif. This was the original reason for making the BLU team the protagonists of the story. However, I quickly realized that choosing the BLU team fit in with the whole "underdog" angle I was going for: one Team against an unstoppable force. (Kind of like 300, except it's only 12-ish.) If you've seen the Meet The [insert character here] promotional movies for TF2, the BLU team are always mercilessly trounced by the RED team, and this concept is often carried over into various Youtube videos. In the actual game, both teams are evenly matched on most maps, and no one (usually) minds being on a particular team. Between matches, the teams are shuffled around so often half of the players from one team are swapped with the other side when the map changes.
For various reasons, I am taking a lot of creative license with how much the story of my comic resembles the game, how much it resembles the Meet The ... movies, and how much I'm just making stuff up. For the internal consistency of my storyline, BLU Team consists of exactly one member of each class, whose names actually are "Scout", "Engineer" et cetera, and the same goes for RED team. Hence the need for The New Guys to step in and help out...
Incidentally, this was the very first scene I ever posed with multiple characters. That's why it looks horrible to anyone who's done a lot of posing in GMod, and isn't that noticable to anyone else. This scene was also posed before I had to reinstall GMod, so this scene has actually been lost.