They're not interchangeable in the sense that both will work on either of the systems for which they are designed. That is, the plates made specifically for the 501 and 503 Pro Video Heads (which are part of the Manfrotto RC5 family) are too long to fit into an RC2 receiver for which 200L plates are designed.

However, what it seems you are really asking is if the 200PL plates designed for RC-2 system receivers will mount into a 501 Pro Video Head. (Only the plates have the "PL" designation. The receivers do not have a "PL" designation.)

The RC2 plates might fit in the channel on the 501 Pro video head receiver. But the locking system on the 501 is completely different from the RC2 locking system and there would be no way to lock an RC2 plate, such as a 200PL or 200PL-14, onto the receiver of the 501 Pro Video Head.

You can always "stack" a 200PL plate onto the top of the 501PL plate (which should come with a 501 Pro Video Head) by using a Manfrotto 323 RC-2 receiver or one of the many 323 RC-2 receiver clones sold by third parties. 501PL design allows it to fit into the 501 Pro Video Head with a plate adapter such as the 323RC-2 attached to it.