(i) Generally yes, but read the SL/SB for full details, there is pages of it for every conceivable combination of engine/EEC/FADEC etc. It is undesirable not to mod both engines at the same time because of the differing accel/decel rates a mix would give.
(ii) I dont think so. I would imaging the perf differences are negligible or exceed current cert perf, eg "a faster engine deceleration rate during Refused Takeoff maneuvers".