Favorite Mexican restaurant is El Gallo Giro in Kuna, about 20 minutes out of Boise. When you go into a place for lunch on a weekday and it's filled with migrant workers, you know you're in a pretty authentic place
As for breakfast stuff--I'm not sure about breakfast tacos
but Los Bedos on Fairview throws down about the best breakfast burrito I've found. They're delicious, cheap and gargantuan--I'm 6' 4" at 240 lbs, so you know I can pack the food away--if I split one of their burritos between two meals, I'm absolutely stuffed twice. And they cost something like $2.75.