Wonderful figures! Where did you get them?
As for rules to use, then I have a couple of suggestions:
Dirtside II / Stargrunt II from Ground Zero Games (
LINK) - Both are rather detailed, but rely heavily on "design-your-own" which might put some off. The games are always cited as "the most realistic" but either way, you could easily plonk down these figures and play using these rules without having to add much more. And they're free!
Future War Commander (
LINK) - Again, due to your fire-team basing then this game could also work a treat. It's based on the Warmaster series of games from Games Workshop, but heavily modified into a game that is similar but different. It has a "design your own" section, but the lack of armour for your forces may or may not be a let down for this game. I've played it more than SGII/DSII, so I might be biased.
And then there's Wildfire, as mentioned by the Flying Squirrel.
Otherwise, you could easily use most games - like Combat Zone - and simply let each fireteam count as an individual in that game, but simply give them more durability for the game. In Combat Zone, you might simply count a squad as being Gang Leaders and then play the game as is.
Hope the suggestions could help