is it plausible to put 26x9x12 on all four rims?
I would say yes, they certainly are not too big. As fare as having all four tires the same width? I have seen people do it without issues, may feel a little different?
On my previous ATV, I ran 9" front and 10" back, no issue, not equal in width, but pretty close.
I personally will be going 9" wide in front and 11" in the back. 12" if I go with Original Bighorn.
The dealer I bought my Kodiak from had one set up with 26X9X12 front and 26X12X12 in the back and claimed it would not void warranty. Warranty would not be void as long I not go more than 1 size bigger in diameter.