Like you said, there is nothing on Futaba's site about it. Only way to know is to try, so if you've already got one try it, or ask your LHS if you can try one before you buy it. Otherwise just order the short crystals from Tower.
On the Tower Hobbies site it says you must use a short crystal in short crystal RX applications and cannot use the short crystal on RX's with long crystals; so it is probably vise versa.
I'd just order the short crystal myself. $13 is cheap for a crystal set.