well if you build for 12's now, your airspace is going to be off for the 10's.

You dont use multiple size or multiple model subs cause not every sub plays the same frequency the same way. Even with the same drivers 1 10 and 1 12 they will play say a 50hz signal a differnent way causing cancellation. The cancellation can cause your system to sound horrible or the cancelling signals will cause it to not be as loud.