I've always viewed anything and everything published by GW/FW as merely guidelines... which, naturally, have to be followed in a tournament setting, but if you and your friends want to mix it up a little, why the hell not? Make up a non-standard unit to represent something that isn't normally fielded by an army, but is there now. Can't have a certain kind of unit as an ally? No problem. For example, if you bought a box of Dire Avengers just to paint, and have a Space Marine army, take the Dire Avengers as an Elites choice ally if it's just playing with your friends. The Eldar are known to support Imperial war efforts when it suits their purposes. My rule of thumb is to let someone play it if they want to, as long as it's not like, "I'm paying five points for this piece of wargear I made up that inflicts instant death on anything it touches!" or something equally horrifically underpriced.
On topic though... Armoured Company is a hell of a lot of fun to play against. It's a challenge, because your infantry don't have their normal roles to fulfill-- it's entirely a question of figuring out how to hunker down and not get blown to smithereens and still take down that armour. Incidentally, I punched up the cost of that armoured company box, and assuming you armed each as a standard Leman Russ with a pair of heavy bolters, a lascannon, a pintle stubber, and extra armour (which is essentially the standard configuration now-a-days), you're hitting 1820 points alone. You'd have to figure in the fact that you need a Tank Ace leading the squadron, and more likely than not, you're going to be mixing up the configurations to include at least a squadron of Demolishers or Destroyers (or, if you've been paying attention to Forge World over the last couple months, a Leman Russ Annihilator squadron, which eliminates the need for anti-tank infantry support...)
All in all, it's a highly beatable army, and it does take a bit of skill to play well due to the completely different nature of it.
