Keeping the IBs and mains together makes very good sense.
12 x 15"s sounds like a great plan if you have the available volume in the room(s) behind.
Remember that you'll have to multiply Vas (for a single driver) by the
number of drivers x a minimum of
4 to maintain sound quality.
(12 x 4) = 48 x Vas. This is a safe
minimum and more rear volume is better.
6 drivers could be run in sets of 3 x 4 Ohms in series for 12 Ohms per channel on a dual mono (stereo) amp. A nice safe load for continuous high levels so the amps would be cruising.
Or the 2 sets of 3 could be paralleled for a more normal impedance of 6 Ohms with the amps bridged. This wouldn't make much difference in the real world unless you like claiming impressive numbers of watts driving your IB.
Opposed driver manifolds are a good idea to minimise mechanical vibration.
Though I can't help feeling you are going to sense rather more of the acoustic vibration with that much woofage!
