First, to the servos 6v is 6v, no matter if it gets it from a nicad, nimh, or regulated lithium so the torque and speed will be the same for a given voltage.
Now, about that NiCad only warning. Back in the day NiMH batteries had a higher internal resistance. That means that under load the voltage would drop off, sometimes to dangerous levels (especially with 4 cell packs that were easier to drag the receiver down to reset voltage).
Modern NiMH cells don't suffer that issue.
So fly happy
