[javadog]

It's unlikely that you will damage the new rotors with a set of pads that you have sanded to a thinner thickness. What is more likely is that you will probably not sand them evenly, which is very difficult to do by hand, and you'll end up with less than perfect contact between the pads and the rotors initially. It has the same effect as taper wear on the pads.

I'm not a huge fan of mixing pads of different brands or types, either front to rear or on the same axle. You don't know what the friction coefficient of the pads might be and you can upset the basic mechanical balance of the braking system if they differ in a substantial way. It's not a big deal nowadays, given the near universal application of ABS, but it's still not ideal. It's not something you would really be bothered by if you're just poodling around town at low speeds, but I don't know what type of car we're talking about here or how you use it.

It's possible that the old rotor, or maybe more likely the hub on that side, could have been damaged in the crash. It's definitely something I would check. Warped rotors are probably less of a likelihood than instances of uneven pad deposition on the rotors, unless the rotors that we're run previously were of really poor quality.