Does RMS Keyboards require an 88-key keyboard?
Technically, no. You can run RMS Keyboards with any sized keyboard. But, keep in mind that every score is different. While a 49 or 61 key keyboard may get you thru some songs, other songs may call for the lowest or highest notes of the range and you may end up having to transpose that short keyboard on the fly. With an 88-key keyboard, that's never a factor.
Look over your scores carefully ahead of time if you're considering using a shorter than 88-key keyboard.