Some mount them at the seat belt point on the a pillar, but it is painful there, so I would mount on the screen pillars as you say, anyhow you can, hard to say without knowing exact speakers.
The gain on the amp is just like a electric tap, it lets more feed/sound through from the headunit, as the amp runs at a set increase the more you put in the more you get out, setting the gain too high will likely result in distortion at higher volumes, setting too low and the headunit could distort the sound before passing it on to the amp for the amp to increase to the level you want, roughly 13 to 1/2 turn on gain is a usual figure, but just experiment.
Although you like Alpine stereo gear, your ears may well like another make speaker wise.
The speakers have but one function, to sound nice to YOU, so buy irrespective of make, other wise you will always wonder about the better sounds you may be missing.
If alpine speakers sound best to you by all means get them, don't buy ANY without hearing them, as it's amazing just how different seemingly similar speakers even of the same make type can sound.
A lot depends on what you like listening too, but remember EAR DAMAGE done now with super loud stereos can be PERMANENT, for the rest of your life, don't waste your hearing just to keep up with the lads, cos you will look a right dick with your hearing aid if you do due to excess noise.
tabetha
tabetha