Will 26X 11.5 ET Streets mounted on 10 holes fit under a fox?

They will fit but you won't get the full potential of the tires. I run the 26x11.5 ET Streets and I used to have them on an 8" rim. Switched to a 10" rim and made a big difference. If you move the sidewall of the tires out it gives you a bigger contact patch and the sidewalls react properly instead of distorting like when you have them squeezed on an 8".