You can mix brands of same type rectifier tubes, they don't have to be closely matched for rectifier. It won't affect noise or hum if the PS smoothing is good. Each rectifier handles one side of the power tranny secondary. So after rectification and smoothing the PS voltage and impedance is an average of the two rectifier tubes' individual outputs. Variations in efficiency or impedance of the two rectifiers will average out.
A vintage WE tube probably costs so much because it has lousy efficiency and higher impedance compared to a more modern tube. Higher rectifier impedance makes the PS impedance higher, making the amp warmer, smoother, less damping, less control, less detail. That vintage tube sound.
Replacing one WE rectifier with a modern low impedance rectifier will lower the average impedance of the PS, adding more damping to the amplifier, more detail, more solid bass, etc. You may not want that, so then you need both rectifiers to be higher impedance and you gotta pay up for the old technology.