I recently installed the same fenders (SKS) and Schwalbe Big Apple tires on my Downtube Mini. They definitely do fit, however the rubber "accordian" over the brake cable between the calipers does rub against the fender. No big deal, as it doesn't affect the braking. I used all the orginal mounting hardware that came with the fenders, but I did have to cut about 1/2" off the rear struts. I even installed them without taking the original tires off, as I was still waiting for the Big Apples to arrive.
Have another go at it. Don't be afraid, you won't break anything, and it will be a great learning experience, giving you confidence to go on to bigger and better things. Good bicycles are amazingly tough and hard to break!
