I would stick with NiMh or LiPo packs. NiCds are old school, and really don't have the capacity or discharge current capabilities that the other packs do. Plus NiCds also develope memory, and I don't even think you can go out and buy a sub-C sized 7.2 volt NiCd pack anyway.
LiPo has the advantage of being a smaller and lighter battery pack, but they require special care when charging or discharging. Weight is not always a concern with these trucks anyway. A little extra weight, if you have room for it, is a benefit most of the time. Better traction. LiPos require a charger and a balancer.
NiMh packs are cheap, readily available, and just about any charger you buy today will work with them. Not always so with LiPos. You really don't need anything special to charge NiMh packs. They are reliable, trouble free, safe, and proven.
If you are curious about LiPos, read this site. It's very good, though tailored to helicopters, but it doesn't matter much. The care and feeding of a LiPo will always be the same, no matter the model you use them in. Some good information can be found there. It may help you decide if LiPo are for you.
http://www.rchelicopterfun.com/rc-lipo-batteries.html
Voltage is sort of a personal preferance. It can also vary with what motor you want to use, what speed you want your truck to go, and gearing. I think if you go with NiMh cells, I'd stick with 7.2 volts. It's the most common battery pack out there for NiMh cells. You can find them anywhere, and they are cheap. They also fit the stock battery box and hold downs on the Tamiya trucks, without any issue. A 7th cell, depending how the battery is built, can cause fittment issues. If it's added to the end of the battery pack, the 7th cell will prevent the pack from fitting into the stock location on a truck. If the 7th cell is on top the other cells, then the pack will fit, but you might need to find a different way to strap it down. Adjustable velcro strap, for instance, instead of the stock battery hold down bar. The extra cell will give you a tad more voltage, (more power, and speed) but it's really not needed. These trucks are not power hungry models. They'll run a long time on a cheap 7.2 volt pack, especially if you are running a higher turn motor, that draws less current, like a 55 turn.
If space is an issue, I'd suggest LiPo packs. They are made to be light and small, or else large with huge capacity! If you want to run a truck for 2 hours non-stop, I guess you could get a LiPo pack with 9,000 mili-amps of capacity. The biggest NiMh packs I've seen are about 5,000 mili-amps.
Where 12 volts is nice is if you have some really slow motors, or some faster motors with a lot of gear reduction. You'll have a ton of power that way, but added complexity, since now you need a 10 cell NiMh pack, or else a 3S LiPo (comes in at 11.1 volts). Or if you like a lot of lights on your truck, you can wire more LEDs in series on a 12 volt pack then you can on a 7.2 volt pack.
If this is your first truck, you can just keep things simple and go with NiMh 7.2 volt packs. Later on, maybe try a 2S LiPo, which is 7.4 volts. The transition from one to the other is really simple. Just make sure you have a voltage cut-off for the LiPo.
The receiver will be okay, with any voltage battery pack you decide to run on the speed controller, as the speed control will regulate the output voltage to the receiver, with it's internal BEC. So no worries of burning up the receiver or servos.