ATE is quite possibly same as OEM so should be OK. Mixing makes of pads without testing is not exactly a safe thing.
The balance of the brakes is pre-determined by Volvo in the design of the braking system. In this, they expect particular characteristics of each part to work together. Mixing different pads front and rear can be dangerous.
Each manufacturer has their own compund for brake pad material. This may be optimized for any number of situations. Some may work best when cold (for stop-and go city traffic) others may work best only with high temperature in the brakes (for high speed or mountain driving). This would also apply to the rotor steel, but in a more minor way.
It's probably safest to figure out what type will work best for you and keep the balance front to rear as designed.
Scroll down this link for a good summary;
Brake Pads / Rotors Technical Specification - How Brakes Work - FAQ