if you have a quad with 25/10/12 tires
and you but tires of the same size but diffrend brand will they fit the stock wheels?

 

Yes.
The thing you need to watch is the last 2 numbers, 10/12. The 10 is the width of the tire and you can go wider but you could have problems with clearance for steering, shocks and frame. The 12 is the diameter of the rim. This cannot be changed without changing rim size. The 25 is the height of the tire, you can also go larger on this but will affect your power and you can have clearance problems again. Hope this helps.