Are these arbitrary to the manufacturer or is there FAA guidance/mandates?

I'm not sure what you're asking. The kit maker or the guy who drew the plans reports the stall speed he got on the prototype or that fell out of the formula when he designed the airplane. The actual stall speed of each aircraft depends on how it was built. That's why there's a test periods of hours to fly off. Your supposed to spend that time establishing the actual performance numbers, including stall speed, for that particular airframe.

Fairly certain only certificated aircraft have limitations on stall speed, if that's what you're asking

There is no 61 kt limit. You can build a plane with a 200kt stall if you want.

Above posts sum it up well.

A Lancair, RV or similar are always true to manufacturer book numbers. The biggest error is actually the static port error which affects the IAS.

To be frank though the IAS on that plane could be out by a few knots but whatever the static error is is unlikely to change. Even then a knot or two is hard to pick.

I have to say that my IAS is very accurate after doing some serious static error correction, but this may not apply to all.

That's what I was asking Charles; thanks