Well Mammoths don't like being after anything so I would do it first. I would have him do a mastotron and FFF though. Mastotron is way more versatile and can get all the mammoth tones if I'm not mistaken.
Cheers.
He's is building both circuits with an input pot, so the pedal will be usable after others, with actives potentially too.
I was actually thinking Mammoth first simply so I can use the FFF as a lead guitar tone and either brighten things up or make it crazy, as the FFF is pretty good at oscillating craziness.