I have only been playing for a short while, but this has been my initial loadout,
4x riflemen - carrying Assault Rifle (3 mags), 2x frags, 2x smoke. Also IR goggles.
1x gunner - carrying Machinegun (3 mags), 2x frags, 2x smoke
1x sniper - carrying Sniper Rifle (3 mags), 2x frags, 2x smoke
1x grenadier - carrying Grenade Launcher (6 HIT mags), + pistol (2 mags)
1x shotgunner - carrying Auto Shotgun (sabots), 2x frags, 2x smoke
Everyone carries a Medikit.
The soldiers pair up, with a rifleman accompanying someone with a heavy weapon. They move in two-person teams. If one is injured then the other heals them with the Medikit.
Where possible, I set up an initial position, lay down smoke as a shield, and try to encourage the aliens to come to us. Usually once you take down one or two most of the rest come running. If they can come running through a nice narrow doorway that's ideal. When they're all gone, we chase down the last few.
When moving around, I try to always have people ready to do multiple rounds of reaction fire - at least one in each pair.
This has worked pretty well thus far, I'll see how it goes over time.