You can get firewire with multiple conductors, but it would probably be special order. A lot of the big distributors have drop-ship agreements with the manufacturers, so the cable comes straight from the factory. Elk recommends cat6 for the keypad, so I would go with their recommendation and forget the firewire for the keypads.
For the smokes, power, and horn, stay with the firewire. It's been ages since I priced 2 conductor firewire, so I don't know how much difference there is between 2 and 4, but if the difference is not to great, go with the four. As you mention, you can always use the extra conductors in a pinch. It's like station wire used for wiring contacts. Contacts only need 2 conductors, but the difference in price between 2 and 4 is so insignificant, it's easier to just use 4 for everything.