Never thought it would be that difficult to explain!
It's not that difficult, I had just imagined doing the explanation with a few examples, however I'm away for the next few days and I won't have time ti do the long version before I go so here goes the short explanation!
When you select multiple missiles of different types to fire in a single turn, missiles are not
fired in the order they are selected, they are fired as they are found in a search Left to right on the top row, left to right on the middle and L->R on the bottom.
Thus if your top row is all type A, middle type B and bottom type C you can not fire a C followed by an A.
If you arrange missiles as I do, you can almost always fire any combination you want in any order (ACA, BAC or whatever).
I hope you followed that.