I don't like RNG for customisation when it already costs so much in time and money to do it. It's not like your body shop is going to depend on chance as to whether they can lift your car and fit a new suspension system and fat tyres?
As you said, the Argo is capable of limited customisation so no engine swaps.
The hardpoints are just a way to show that when they designed the mech, they ran power lines, control runs and cooling systems suitable for a certain type of weapon. So energy hardpoints would have a lot of power and cooling connections while ballistic hardpoints might not need as much cooling but require more solid mountings, etc.
Changing those parts around is slightly out of scope for what is in the Argo mechbay so that is what hardpoints might be showing. {It sounds good anyway.}
Agreed.
Looking at customization from the outside (i.e. without taking the TT's rules into account) and realistically speaking (yes, yes, I know), changing a ballistic weapon to a energy one (or putting any weapon where non has been before) would be a _massive_ undertaking.
Looking at replacing an AC/5 with a PPC as an example, as this is one of the most common custom jobs in TT, you first would have to re-wire the entire mech, so you get enough power-lines to the PPC.
Next you have to redo the cooling system, since that PPC is a massive heat-hog and will need 10 times as much cooling as the AC - and you can't just add lines either. The cooling capacity of your mech is limited, so you have to cut things elsewhere to make it work (same with the power lines, btw).
Now you have to modify the internal structure, so it can actually _hold_ the totally differently shaped PPC and remove the ammo-bin and the ammo-feed system.
Next comes the re-programming of the targeting system, since the PPC's "shot" doesn't drop due to gravity and has a different muzzle velocity/different drag rate in an atmosphere. You'd also have to reprogram the mech's gyro, because you changed the mass distribution on your mech and have replaced the AC with a weapon that doesn't have recoil (or at least a lot less).
You'd also have to stress-test all modifications, to make sure they hold up in combat, first one by one, then all together.
Looking at the above, it kinda becomes clear why omni-mechs were as big an advantage as they were in the lore.