21 speed is usually a three speed front chainring hooked to a seven speed cassette or freewheel
A seven speed dérailleur chain is 1/2 X 3/32.
A 40 chain is 1/2 X 5/16
No. The chain is commonly called "multispeed chain", it is application-specific. You must use that size chain if you want to keep your gears working properly.