OK. You asked a lot there... The ZR360 is a much older amp than the KX400. I'm not familiar with thei specs so you can check those for yourself at www.kicker.com. As for which one to use, you need to make sure the sub has the correct DVC configuration that will get the max power from the amp. In other words, if you have a dual 2 ohm sub, then you want the amp to max power at either 4 ohms or 1 ohms. If the sub is a dual 4 ohm L7, then you want the amp to max power at 2 ohms.

As for the front speakers, I'm assuming you have a 96-00 Civic based on your pic in the sig. The stock size is 6.5", but aftermarket 6.5" speakers usually have much larger magnets and will need a little modification for it to fit.