The key considerations are realism and playability (in this case, bookkeeping). Ammo falls in the optional category of the Rules Matrix because it is something I acknowledge as a valid concern but won't advocate specificially for it, while there is a limited, objective need to use the rules.
Optional is fine/cool. As long as across the board equal option is there.
We eye-balled something of an "encumbrance" system where each frame could only carry so much weaponry. It's a fudge because we didn't want to get into too much detail about what each trooper could carry.
Encumbrance is based on two factors. First is the sheer weight of an object. The second is, where the heck do you put it? Powered troopers would probably not have to worry about the first for o ammo but the latter would come into play.
Ah, yes, but another reason for the question (though not the prime one) is that you have listed in the Advance Rules that '... carrying excess grenades does not hinder ...'
The third thing is simply a matter of doctrine as to how much ammo or firepower each trooper carries.
We didn't refine the rules but your question is valid. A fire action should allow you to loot a body up to your FCL. The question is can you exceed FCL?
FCL= Full Combat Load yes?
Highly detailed and extremely optional rule(s ) should possibly address this.
IMO a possibility (take UNE troops)
Pathfinder P.B.A. capacity
FCL: Blaster (or RAM Laser), 2 F-Grenades, Target Designator (field), Rocket
pack (field)
Max Limit: Hand held long arm, pistol, 4 grenades, a backpack (or rocket pack), Target Designator
Paladin P.B.A. capacity
FCL: Blaster (or RAM Laser), 1 F-Grenade, 1 K-Pulse Grenade, (or a Grenade Launcher)
Max Limit: Hand held long arm, pistol, 6 grenades, a backpack or an indirect fire weapon (like a seperate grenade launcher)
Max Limit for support weapon (Hvy): Support weapon, pistol, 4 grenades, backpack only or portable pack fusion for weapon (eliminates the need for a E-Link figure), or 2 loads for an o type weapon. (Takes 2 firing actions by the figure carrying these reloads to use a reload. First one to remove the reload from the carry position, second firing action to reload. This is true for either the primary hvy weapon figure or another figure assisting them.)
Max Limit for support trooper to a Hvy weapon trooper: Support weapon, pistol, 4 grenades, 4 reloads at the ready and backpack only or portable pack fusion for weapon (eliminates the need for a E-Link figure), or 2 loads for an o type weapon. These 2 extra loads require 2 firing actions to used, 1st one is to remove the reload from the carry position, second one is to either reload the Hvy weapon or hand the reload off to another.
Ironsides P.B.A. Capacity
FCL= Ram Laser, Grenade launcher, 4 wrist rockets, 4 FTG Grenades,
Max Limit: Hand held long arm, pistol, 6 grenades, a backpack or an indirect fire weapon (like a seperate grenade launcher), 4 wrist rockets, 2 reloads for an o type weapon.
(Takes 2 firing actions by the figure carrying these reloads to use a reload. First one to remove the reload from the carry position, second firing action to reload. This is true for either the primary hvy weapon figure or another figure assisting them.)
Very Optional Rules
The backpack can carry up to (blank) equipment. It takes 1 firing action to remove backpack (or gain entry to backpack by another figure), 1 firing action to remove item from backpack, 1 firing action to replace backpack (or reseal backpack by another figure), 1 action to reload, install, prep the item for use.
A backpack that is not replaced or resealed will lose items. A check for what items are lost from backpack is not done until the pack is searched/used again. Roll 1 die for each item on a 1 or 2 the item has been lost.
Carrying/wearing more than the above listed items causes Encumbrance issues.
Any fire action is +1 by an encumbered figure
Carrying items equal to or more than ½ as much more, has the following penalties; -1 movement, + 1 to fire
(example a Pathfinder P.B.A. carrying 6 grenades and 2 pistols or 8 grenades)
Carrying twice the number of listed items penalties are; -1 movement, +2 to fire
Carrying 3 times the number of listed items penalties are; -2 movement, no fire allowed.
Dave Chase