I have not had any problems with those scratchers falling off. I also leave them down way more than what I should, but I got them, so why not use them!
I'm not sure if you are going to run all of your wheels or not, but I removed my front wheels and mounted them there. As a rule of thumb, you want to mount them as far forward on the skid as possible, if you can get them in front of the wheels, that's best. However, they are really hard to hang up with the wheels there, just doesn't work the best.
Just put some locktite on them and you should be good to go. Make sure you torque them down, I can't remember the specs off the top of my head. I think the package states what they should be at!