It really depends on whether you're going to be braking hard with the pedals. If you're running with no brakes, I'd definitely suggest not doing it. If you've got brakes, and aren't given to skidding or hard back-pedalling, you might try it. Fixed hubs are pretty cheap these days though (given they bikes you have, I'd bet you have an old flip-flop hub with a lockring thread on it somewhere; they were VERY common on 50's/60's road bikes, and I've had 3 or 4 in my time)