I think 12 15s free aired would sound good... Not saying it's not impressive, because it is! but if you have 12 15s, you're going to be loud, just about no matter what they're in.
Yeah. Not really a good idea to mix and match amps like that. 2 nice mono blocks would be good for your setup. If you're looking to run 2k+ rms well, look into AudioQue, they are cheap and very good quality. I think a 2200 rms amp from them is $389 shipped. Might be a pain wire up 10 subs to one amp. What are is the ohms of the coils?
Info on HDC3s
Order form
The HDC3 are beasts. The standard copper coils are good if you want to run around 1500rms, but much more than that, I would go with aluminum coils just to be safe. AQ has great subs for amazing prices with great customer service. It has 3 inches of usable throw. I have my SDC2.5 in a 4.1^3 ft box tuned to about 35-36hz. 4.5^3ft tuned to 36-ish hz would be good. Sounds amazing. The HDC3 will get a lot louder with more power.
HDC3s would get loud off of 2500rms, that's 1250 rms a piece. Wouldn't have to worry to much about over powering them.
If you ran 4k rms to 2 hdc3s, I'd get aluminum coils.
With running 4k or more rms+ I would run 2 alts, 2 220 alts and 2 big bats in the back would run smooth. I've seen people run 3k each to HDC3s with copper coils. So with Aluminum coils, watching your gains and clipping, you would be fine.
Stop using "an" to mean "and," it's not that hard to push the "d" key. If he wanted a true 2k rms system, I'd go with the AQ 2200 and a Level 4xl D2 coils, if he doesn't want to go with the HDC3 18. (I'd go with the HDC3 though)
If the prices are right on the 21" they would move a bit of them. IA probably moves a few because they are one of the few companies that do for a fair price. Also, being as it's DC, they would move a SHIT ton of 8s...