Estel : sure, a "chance" or "weight" could be added as a rule, making all weapon types of a specified group have an extra chance to be selected. Another thought is to add this to specific weapons, but with a negative value, i.e. making weapons more rare. This way we could add some nasty surprises which might pop up once in a while
So to sum up the comments : we want the main guns to be lasers or missiles, and we want the secondary weapons to be selected from all possible weapons, with a significant chance increase for getting ion/beam weapons.
If this is the case and you want to test it out, we can use the current tool and change the assets (i.e. create copies of the ion/beam weapons in order to make them more likely to appear), creating a new version with the desired randomized content.