Originally Posted by CliffordK

NO

Vertical dropouts are designed to have the axle tight against the dropout.

Older vintage steel bikes with horizontal dropouts can be adjusted that way, but not the new vertical ones.

As far as tires, it is perfectly fine to mix tire sizes. 28mm in front, and 25mm in the rear. There is also debate on whether the larger tire should be in front or back. The larger front tire will give you more shock absorbing for the hands on your bars, and be the first to impact obstacles. On the other hand, the rear carries more weight.