I've found that 29" radius is the minimum I could go with fixed pilots and Kadee couplers. Anything less and the locomotive will force the car to derail. This is on my 29" radius passing siding.
My mainline radius is 33" and I've had no issues, either between locomotives or locomotive/freight car.
The Flyer coupler may have a greater range of motion than the Kadee, but I haven't tried it.
Rusty