[Effect
nickname = my_new_weapon_effect_proj ; the nickname of the effect (needs to be unique)
effect_type = EFT_WEAPON_PROJ ; the type of the effect (for weapons, this stays the same)
vis_beam = ci_laser_03_beam ; the effect from weapons_ale.ini that is used
vis_generic = li_cruiser_maingun ; another effect, also from weapons_ale.ini
If you look for the maingun's effect (li_cruiser_maingun_proj) and it tells you that it uses vis_generic, then put that in. Just keep the vis_whatever the same as the effect it's taken from - you can have different types in one effect.
A few years ago I made pictures of all the weapon effects - you can see them
->Here<-. The name of the picture is the effect's nickname from effects.ini - should be helpful
So now you should be able to mix and match until you find something you like.
As for your script problem, I still don't know why it won't work.