Tubes are very flexible and expandable so many sizes can be made to work even if not ideal but I tend to agree with the above suggestion to get each participant to carry at least one tube that fits their own bike. Supplemented with a patch kit or two and a willingness to share that should be sufficient. But I've used 700c/27" tubes successfully in 26" MTB tires, 20" tires, and even in a 12" trailer tire. Presta valves can be used temporarily in rims made for Schrader valves (esp. if you protect the valve stem area with some tape), but not the other way around. But make sure that you have a pump that will work for both types of valve.