Excellent choice in choosing to stay with the 6 volt types Nimh/Nicd and it is most definitely not overkill to use the 6v (5 cell) versions of either with your 2.4 system.

There is another good reason why the 5 cell over the 4 cell (4.8v) types is a good choice and that is simply put, speed! You will find that there is a very slight speed advantage in the servos. Now why is that an advantage to me you might say, I'am not fancy aerobat or perhaps just a new flyer. Well yes its true a relatively new flyer will likely never even notice the difference, However I have seen it time and time agine a newbie will do perceptively better and smoother with faster servo response ever though He may not notice. Indeed that slight servo speed is an advantage to pilots of all skill levels. Back in the day when electric servos were first used they were so slow it was like having 100% expo on at all times.

So even before 2.4 there was good reason to choose the five cell types over the four cell types.

John