Will the ROM for the MPX200 work on the MPX220?

No, they are two separate ROMs. With Windows Mobile devices every device will have a unique ROM. No ROM can be flashed from one device to another. Now if they're identical devices with different branding, like the various i-mate and Dopod products, then you can interchange the ROMs. But the MPx200 and MPx220 are very different hardware and software.