yes, FF-1 were used in the prototypes, now we've got similar feet, 6.5mm tall, but made from a bit different material that is softer. Those Chinese feet are made of plastic which isn't perfect.
We have tested FF-1, FF-2, FF-3, FF-6 and FF-9 and decided on FF-1 since the performance was significantly dropping when moving from 6.4mm feet to 4mm tall FF-9.
All feet with 8mm hole should fit - it's just a round 8mm hole. The only thing to note is that the E minus D should not exceed 7mm as it might mean you'll be touching the motherboard with it. It's not a problem, since you can shave some plastic off, but that's just heads up.