As Chad and Josh have said these designs are to get people started in the hobby.
Trying your own experiments is half the fun. A smaller plane would just use smaller motor (weight) and battery. I say go for it and have fun.
you shouldnt have too much issue with simply printing the plans out at a smaller size. the only issue I see is weight. You may find the wing loading (weight of plane to the area of the wing) is too great. You may have to remove the paper from the Readi-Board to balance things out. I know the paper adds a lot of regidity to the foam but it is also very heavy and can come off very easy if it gets wet. You may want to consider removing the paper from the inside surfaces to reduce the weight of the plane, but keep it on outside to keep that regidity. Just be warned that if you remove the paper from only one side the foam sometimes wants to curl a bit. Once everything is glued together it shouldnt be an issue though.
You can go with a 3mm depron which will keep it lighter, and a profile build as FM suggested will shed a bit more weight. You will need to tape the hinges instead and I wouldn't use hot glue but UHU POR or even Gorilla glue to keep it as light as possible. you could probably go to 60% or even 50% if you're very careful about the weight and electronics you put in it...
A thought, and mind you I have never tried this, would be to take the paper off one side of the foam (inside of wings/fuselage). You would maintain some rigidity, reduce weight, and i think it would be sturdy enough considering the reduced surface area. I may give this a shot with the Spitfire myself, put that damnable blue wonder in my parts bin to use.
Scaling to 60% is a perfect fit for 3mm Depron of you want to go that small and have access to 3mm foam. I've built 11 planes so far using a 60% power pod. Check out the 55% Mini Racer thread for my power systems.
I have also built a 65% Old Fogey (simply scaled down the pdf plans) out of 3mm depron and put a 10g motor on it. It just weights 120g with battery and as I have increased the wingspan it can fly really slowly but I still have to put a bigger rudder on it because it flies so slowly that the rudder doesn't make any effect.