In order:
Yes, the rules are in StratOps. Anything can try to use any guns to shoot down CapMissiles.
Both are options. The numbers aren't easy, so I advise using the full warload to get more dice on your side.
Also, don't forget the main method the SLDF used to protect their big ships from missiles: using their smaller units (whether they be destroyers, corvettes, DropShips, or fighters, or any combination of the above) to destroy the missile units outside of direct launch range. It's the same method they used to protect their big ships from fighters.