You can't force the player to take a certain loadout... this would be a wonderful thing to be able to do.
However, you can tell the player in the briefing to limit himself to the weapons specified in the briefing text because you can put features into the counter to automatically fail and end the mission if say he fires 7 TASMs when you said his ship has only 6 TASMs.
If you need help, I'd PM tlam about it or perhaps he will drop in here himself when he has a moment, since it was his very intelligent find on how to do this.
