How to choose a flange?
Using three information
Pitch Circle Diameter (PCD)
Number of Bolt Holes
Pitch Circle Diameter (PCD) is the diameter of a circle that goes through all bolt holes.
Number of Holes matters. Different flanges can have the same PCD with different number of bolt holes.
Can flanges in different standards be used togerher?
They cannot in most of the sizes. Yet, for some sizes, different standards shares the same PCD and number of bolt holes. In this case, they can be used together. Sometimes, people use smaller bolt for flanges with PCD difference less than 5mm to avoid fixing the pipe.
Does outer diameter affect installation?
No. Outer diameter would not affect bolts going through two flanges, only PCD would. Difference in outer diameter will only affect how it looks.