1) when/how do i unlock the YF-21?? You.. Already have it. You get it after mission 2.
2)How do i fire gundam x's main cannon? all it does it charge up when i use it but i cant actually fire it You have to hold the button input. It takes a while to charge, and cancels if you let it go early. It fires when you let go after the X panels begin to flash fast.
And for YF-19, there are two moves I can think of. One of them is where it holds its arm out in front, that's a shield and reduces frontal damage. The other is probably the one you're talking about, where you shoot out sparkly stuff. That's a chaff, though I'm not sure if it works on homing lasers.
If you're going through hell, keep going. - Winston Churchill -