Well for instance, a madcat's ear is a mount for a pod, you can put a missile pod on it, a large or assault weapon, or a triple pod of medium weapons like lasers. The Mauler on the other hand has very restricted slots, you can put missiles in those torso slots but nothing else, while the arms need large or assault weapons (though i might have put some medium lasers in the arms once instead?) It doesn't matter whether they be energy or ballistic. While the four nipple cannons used to be small weapons only, I asked that they be upgraded to medium slots and it was granted, thus the UAC5 mauler and the 4 ERMBL mauler.
While the Fafnir for instance MUST have at least assault ballistic weapons in its two main cannons, nothing less. I put some Tbolts in them and it was awesome. But then everyone vetoed it.
Its pretty much the reason the x4 L.gauss had to go cause the Lgauss is a large weapon.
Every mech has its own set of rules really. We would have to go through them one by one to explain it all and there are still some things that are grey areas where we had multiple devs asking each other thinking someone else knew. I personally wanted large weapons on the partisan but I think it was vetoed due to it only being allowed medium weapons, thus the disappearance of the L.Gauss partisan. Note that pod "sizes" don't really apply to missiles, if it can hold missiles it can pretty much hold any kind of missile launcher as long as it doesn't look completely retarded. Thus the MRM shiva has lrm15 launchers? rather than LRM20 even though it has MRM40s and 30s because the LRM20 launchers looked stupidly huge.
Defender is right in that the rules are just as much for aesthetic reasons as for balance reasons, the mech needs to LOOK plausible.
I recall multiple calls for the canon Novacat with the 6 LRM-15s but there was no way to make that not look totally retarded.