About the best you can realistically do on a KFX-C is a
Kentucky Chicken Fryer. Triple solid AMS with a single ton of ammo, which is sufficient for the job if you're actively turning it off when not needed. Five cERML, four LMG with two tons of ammo, and a pair of jumpy-jets. It's...adequate, ish. Does semi-decent cERML midrange skirmishing, which is what you want for a 'Mech designed to stick with the herd. You
could outfit it with APG and SPL if you want to do the thing where you ward off enemy light 'Mechs while covering the fatties with AMS, but then you have to find an extra half-ton, and also you're dramatically inferior at that job when compared to an ADR-W or similar massed APG spammist.
It's not the worst use of a Cute Fox, but the Cute Fox isn't an especially great 'Mech to start with. Killing two and a half tons minimum on triple AMS plus ammo is a fairly steep price on a thirty-ton light compared to quad AMS on a 95-ton assault. The Cute Fox
does have the benefit of also bringing ECM, which ain't nothing, but the 'Mech is still rather anemic for its weight and mobility.
The PIR-A, on the other hand, is unsalvageable. The absolute
best I can do while retaining all four AMS systems would be something like
this donkey, which is
disastrously slow for a Doomfish and also pretty lightly armed. 3x HSL and eight LMG is, again, adequate-ish, but it's a shorter range profile on a dramatically more fragile 'Mech, it does
not offer ECM coverage, and at that speed you're basically a free kill the moment anything decides they're done with your jank.
Less Donkey here is vaguely better in that it still moves Actual Light 'Mech speeds, but you simply cannot get all four AMS systems in there with adequate ammo. Dropping two kinetic AMS and their ton of ammo for a LAMS at least keeps you in the game, and the spare ton goes into the engine. Even this feels anemic and bad, and given that the entire point of the PIR-A is "IT'S THE AMS ONE", it really feels egregious. Also feels like a double AMS ammo quirk wouldn't go amiss, though even then you'd mostly just have the same dogwater armament with more uptime on the AMS.
A much better idea is to use the profusion of 'Mechs out there capable of running double AMS alongside actual-for-real armaments and then overclock them. Yes yes, all the triple and quadruple AMS 'mechs are running AMS Overload too, but that's honestly overkill against all but the densest missile rain. remember, the point isn't to turn enemy missiles off altogether while you sit behind a rock and cower. The point is to blunt the enemy missiles while you're moving up to engage and eliminate the enemy missile platforms, and paired Overloaded AMS does that just fine while being available on a variety of platforms capable of playing pretty aggressively. I quite like
Lacerator for it myself, but there's a number of non-Heroes that also feature two AMS hardpoints and the ability to mount a Real Boy armament.
Remember: the goal isn't to run the enemy missile spammer dry. The goal is to make the enemy missile spammer
dead. You do that by shooting him with bullets; your AMS systems are only there to make sure you get the chance.