[Suhiir]

There's no way to create a default formations USMC Rifle Company formation at the Fire Team level because WinSPMBTs command structure doesn't allow it.
As you said, a Rifle Platoon (Fire Teams) exists however.
If you get a little creative with the editor you can create a "proper" formation.

Buy a normal Rifle Company.
Delete all formations/units except the headquarters unit.
Buy three (3) Platoon (Fire Team).
Buy a light mortar section.
Use the editor to change the three Platoon (Fire Team) from "Formation Type 3 (Combat Group) to Formation Type 1 (Platoon).
Attach the three Platoon (Fire Team) and light mortars to the HQ from the Rifle Company.
Walla!