if you will be running a single chainring with a freewheel in back and friction shifting or index shifting with a matching derailleur and freewheel (all 7 speeds for example), i have found that compatibility issues are minimal.

chainline is not an issue if you mount a single chainring in the middle position on a triple. you will be able to use all the gears in back.

BCD is defined as the twice the distance, in millimeters (25.4 mm per inch), between the middle of a bolthole on the sypder and the middle of the spindle.

i think most compatibility issues are a complication of indexed shifting. if you are moving to a single front chainring, shifting issues are not involved.