Can I use 25.4mm bullhorn brakes on a 26mm handlebar? I've got a 26mm bullhorn handlebar but it seems like many stores sell bullhorn brakes that are for 25.4mm bullhorns.

Will this 0.6mm make much of a difference? Would it be okay to use the 25.4mm bullhorn brakes on the 26mm bars?
It seems like you're confusing the clamp diameter with the grip area diameter. 

26 mm is a standard road bike clamp diameter (i.e. where the handlebar attaches to the stemp) and 25.4 mm is a standard mountain bike clamp diameter. Your stem needs to match the clamp diameter. The clamp diameter and stem must match exactly. 

The grip area diameters need to match for the brakes (which are 22.2 mm for mountain bike stuff and 23.8 mm for road bike stuff). 

Most bullhorns are typically 23.8 mm grip area, so they can use road bike levers.