Is it ok to mix 23 and 25 tyres
There isn't a problem with it, though you may find your ride quality to go down a bit by moving to a smaller tire (but 23 and 25's don't really have much cushion to begin with so this won't matter too much). There is also a slight change in gearing due to the change in diameter of the tire. Generally though, I'd expect a bigger tire in the rear than in the front due to where you sit. 

In general, you can mix tires so long as they go on the rims and clear the frame + brakes. Here is everything you want to know about tire sizing, including recommended rim widths for given tire widths.