26 x 1.75 front wheel will work on a bike with a 26 x 1.95 rear wheel size? I got a 26 x 1.75 front wheel. Tire and tube 26 x 1.75. Upon further inspection, (I got confused) The existing rear wheel and tire is 26 x 1.95. Will the front wheel size be  okay to use?
Provided the wheel fits into the bike and you can use the brakes, it'll be fine to use.

People run different tire sizes on occasion, and in some cases even different wheel sizes (e.g. 69ers and 96ers which have a 26" and 29" wheel). 

In some applications, its even desirable to put a bigger tire in the back since it carries more weight than the front. This can give some more comfort, e.g. for touring.