It's a force organization thing, Flames/TY depicts company to battalion level formations these would all be using mostly the same vehicles for logistical purposes, unless you're depicting a kampfgruppe cobbled together from a bunch of decimated units you're not going to see a hodgepodge of random tanks together. For example you could still bring a StuG tank destroyer platoon supporting a Panzer IV company, but not a bunch of random singles that you happen to own.
Flames of War, at least version 4, isn't as restrictive as you made it out to be. While you can't field an army that's one Tiger, one Panzer IV, one Stug, and one Puma, it still lets you pick any platoon as a support platoon. Even across factions (so you can have a British tank company and a US paratrooper company). Anyway, I'm interested in any of these less well known games in 15mm.
Glad to hear you say so, I've only managed to play it a good half a dozen times both in v3 and v4 really wished it had a bit more of a community here in Perth.
I plan to take this box for bolt action in 15mm, I already have tanks from world of tank miniature game and I wanted to add infantry, you let the miniature on the large Platform, I can have details?