[timstorey]

If the spec calls for a Cl 125 FF Gate Valve, then use a Cl 125 FF flange.

If the spec calls for a Cl 150 RF Gate Valve, then use a Cl 150 RF flange.

The flange must be appropriate to the valve end, if 150 RF – 150 RF etc.

Actually, the bolt dimensions (BC and diameter) are equal between the two flange classes in 8″. The most important issue is that both flanges need to be FF. As long as the valve manufacturer’s bolt torque requirements are met, the suggested arrangement should work fine.
RF to FF arrangements can cause gasket problems and cracking of the DI or CI FF flange, since RF flanges are typically steel.

Tim Storey