Darren,
Actually, quite the good question! A standard "Assault Helicopter Company" that served as part of the 1st Aviation Brigade in Vietnam would usually have two to four platoons, depending on numerous different circumstances. Usually they would have one platoon of slicks (UH-1D's and later H's) and one of Guns (UH-1B/C and post 1967, maybe Cobras). Divisions all had organic aviation companies as well, which were of the same general format. The only differences being the two "Air Assault" divisions, the 1st Cav and 101st Airborne.
These two units had Assault Helicopter Battalions, each with four companies. More often than not, D company in each of these battalions would be a guns company while the remainder were the slicks used to transport the division's troops.
If you're thinking of doing the 1st Cav, they had two battalions, the 227th and 229th AHB. The guns from the "El Lobos" (D/227th) and "Smiling Tigers" (D/229th) would be used to escort the slicks from their sister companies. They used UH-1Bs through 1967 when the more powerful C models came in-country in large numbers. If you're doing an Ia Drang Valley scene, the B model is the way to go.
If you've seen "We Were Soldiers", the Huey Gunship they use is wrong! Its a UH-1C with miniguns that wouldn't have been in Vietnam for another year from November 1965!
Hope that helps some!