[Suicide Junkie]

As for damaging missiles:
Provided that you hit, damage only applies to one missile; any excess damage does not hit the second missile, but is lost.

With such firepower, the only way I can see you not killing missiles is due to missing.

Larger weapons have accuracy issues (+40, +20, 0, -10, -20), and missiles have a hefty evade bonus (60%)... Base accuracy is 70%, -6% per square.

20mm guns should get you a significantly higher hit rate.
Using a smaller hull will get you some hit bonus as well.

And, of course, if you chose the pacifist culture, they have -50% to accuracy off the top.
In that case, you need the antimissile missiles, since nothing else will hit (1% minimum hit chance for DF weapons).