You want 5 cells. If you found a pack that is 2500mah, then the cells aren't 2/3A cells like your hump pack, they have to be AA's. If there are 4, then you get 4.8V, you want 5 cells for 6V to get the most out of your servos. Not sure where a 5 cell AA pack would fit in a t-maxx without finaggling something yourself.
As for NiCAD or NiMH, NiCad have battery memory issues (if not fully drained, they only charge to the level they were drained too and false peak the charger) and not near the capacity of NiMH. 20 years ago, NiCad was what you got. 10+ years ago NiMH is what we all used (and many still do). Not much of a memory issue, higher capacity for the same sized cell as NiCad and high power transfer rate. The only thing I can think of that makes NiMH not as good as NiCAD is ambient temp. Cooler temps (sub-40F) kill NiMH cell charge way faster than NiCAD. I still run NiMH in the winter, I just keep a close eye on it.
Li* is what is hot now. More expensive, smaller footprint per MAh and super flat discharge rate. NiMH and NiCAD both have a fairly degrading output voltage as the pack discharges. At full charge, a 5 cell pack will have 7V without load. As it discharges, the V goes down until the pack is dead, then it flatlines. LiPo's hold a fairly static output voltage their entire charge, then dump out really fast on you once they are depleted. However, having an output of 7.4V the entire time until the very last little bit means your servos are working as good at the beginning as they are at the end.
All that said, I run NiMH still. You can find 1700mah 2/3A hump packs:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXNMD7&P=7
I thought I saw an 1800mah pack somewhere, but it's eluding me now.
I run these without issue:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVNP9&P=7
One of the few integy things I buy.
. Thanks for any feedback you can give!
