From the couple books I have read:
Below 200-300hz the walls become less effective at blocking sound transmission. Room modes are thought to be augmented by bulking up walls, as you allude to, but they are best dealt with by dimensions and bass trapping. Planning on a single flimsy wall and beefing up others unfortunately is pouring money down the drain.
Square rooms are best set up in a corner firing along the diagonal. Parallel boundaries are bad news. But all in all, best avoided.
Curious to hear what square room folks have to say.