I just got a dawes lightning 1000 with size 700x25c stock tires. I am planning to get a new set of tires size 700x23c. Would this size work or does it have to be 25?
Yes, you can use the 700x23c tires.
Yes. Any further questions are answered here: http://www.sheldonbrown.com/tire_sizing.html
All my bikes.
23 should be no problem. I wouldn't do a race clincher 700x18, but a 23 will be fine. The only problem you may run into is going bigger. A lot of sport bikes can't handle larger than 28 without having the tire rub on the fork or chainstays