Are the fronts and rears same size?
wat i mean is that, is it ok to put rear wheels in the front (and vice versa) - if they r same.

 

yes - front and rear wheels are the same and can be swapped front to back if a stop bolt on the front hub is removed first. However, the front and rear tires are not the same and should not be rotated front to back.
HTH

 

Only the non-track model 18's are 18x8 all around.

Track model 18's are 18x8/front and 18x8.5/rear.