Here is my attempt at a roster:

2 Heavies;
- Soldier
- Engineer/Medic

4 Mediums;
- Soldier
- Operative
- Medic
- Engineer

2 Lights;
- Medic/Engineer
- Soldier

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Naturally, you would have the two heavies at the front, absorbing as much attention as they can muster; where the soldier would be the main source of fire, whilst the engineer attempts to establish a forward-buffer-zone kind of thing.
The four mediums would be the objective-squad, ensuring the objectives get done, once they have supported the heavies to reach the location of the objective. They would of course do what they could to best help them out, and on occasion do some short-flanking action.

The Medium Engineer would mainly be in charge of watching the rear with mines and his turret would be placed up high, or some other strategic place. The operative would spend most of his time trying to help out the front line, whilst keeping an eye out for enemy operatives that might pick off any of the 6 people in the central squad. The soldier too would be right at the front, trying to kill as much as possible, whilst being the main source of ammunition. The medic would play the role of the main-medic, meaning he would stay back, stay alive and keep the rest of his team up and running.

The light team would be comprised of a medic and soldier, who would try to use effective teamwork to stay alive behind enemy lines and thin out their ranks. They would be in charge of hit 'n' run attacks, which would leave the enemy team extremely frustrated, in that they now have to watch two fronts constantly. And this combo ensures that no enemy operative can sneak up on them, because if he turns into any of their teammates, his speed would be compromised and they would easy be able to spot him, since they would know exactly where their team is.

(In terms of defense, however, the bolded classes would be exchanged, and instead of a heavy engineer at the front, you would have a medic, with more health. And at the back, you would have a light soldier and engineer, sniping it up in the cover of a turret, with unlimited ammo and weapon damage buffs.)