My short stories are longer, so I only submitted one short story to MFA programs as my writing sample. I did this and was accepted into one out of two of the programs in which I applied. Keep in mind, though, that my short story reached the 25 page maximum.
Two things to consider: People who read applications have to go through hundreds and hundreds of MFA applications. They put page limits and word limits for a reason. From my experience, they won't read anything, no matter how rich and wonderful it is, anything past the word limit/page limit. They might also be upset that you did not follow their stipulations.
However, I also think that it's important not to go too under the page maximum. I would recommend offering another piece to get closer to the limit. Your writing sample should reflect that you are willing to write A LOT throughout your program.
Hope this helps!