For easy reference, it might be a good idea to determine a few load out schemes for your mecha. Maybe have one for air to air, air to ground, ground support, anti shipping, etc.. Remember your characters are not exactly working from scratch, but, there is leeway to generate your own load outs based on mission requirements.
Check out this site and scroll down to see some examples of what I am talking about. We all know that the majority of missiles our mecha hold are in internal launchers. If you want to detail how many of what type in them, that's okay too, I'll just assume default HEAP barring specifics, but, what I want you to think on are the underwing/body stores. This is where you put your heavy hitting long range firepower.