That would actually be very interesting to see, an earth- or firebender as an Air Acolyte. I don't think it was ever shown or stated that any of the Air Acolytes, prior to Harmonic Convergence, were actually benders. However, I don't see why they wouldn't allow benders to join. Espicially since the entire idea behind the Air Acolytes is that they choose to follow the culture and ideology of the Air Nomads, regardless of their actual heritage. The only downside that I could see is the fact that, often times, a person's bending is greatly entwined to their heritage, so that probably leads to a lot of potential recruits deciding to pass it up or even leave later on.
I don't recall it ever being stated that the Air Acolytes had to be nonbenders, so I guess it is possible. One thing I would add similar to General442's response is that the philosophy of the other bending abilities might run counter to the beliefs of the Air Acolytes, which would then make it difficult for them to join. Earthbending, for example, is about facing attacks head on whereas the Air Nomads and Acolytes practice non-confrontation.
I think that'd be interesting to see. If they were a firebender or Earthbender, they'd probably find it difficult to bend since firebending needs you to be agressive (even though Zuko found another way firebend from the Sun Warriors, he still needed to be a bit agression about it) and earthbending would need you to be headstrong and blunt. It'd be fun to see a spin off comic about it, see an Air Acolyte break through that kind of ordeal and find a more peaceful way to Firebend or Earthbend. That'd be a cool dilemma; i didn't like Tahno, but it was disturbing seeing how losing your ability to bend could affect a person, so if this character lost that, would they stay an Air Acolyte, would they leave?
I feel like there may have been a few bending Acolytes, but their numbers are few and far between. A Water, Earth, or Firebender would likely be less enticed with the idea of joining a culture they wouldn't fit into as much. However, I'm sire that many Air Acolytes have had families of their own and some if their kids may have non-Air bending abilities.