OK, the caldera "can" run on a 3s lipo...
you have to 1st change the fan on the ESC to run off the rx. Stock it gets power from the ESC, and if left that way on 3s the fan will die.
So, after you wire the fan off the rx....you can run 3s. Now, it will be a big difference in speed and power. This will cause more wear, especially on the wheel hubs. a buddy of mine ran his one 3s, and had issues keeping the front hubs on(granted, he is young and bashed the truck hard)
Also, he managed the shred the tires on 3s.
Lastly, on 3s lipo it my cut power if the motor gets to hot. Back to back 2s lipo packs is no issue....but back to back 3s packs my cause the ESC to go into over-heat protection(cuts power to the motor)
So, my suggestion is to only do high-speed runs on a 3s. No bashing or offroad, or you will just end up having to make repairs on the caldera. But if you re-wire the fan it will be safe to run a 3s on the street....for when you have the need for speed
OK, onto the XTE...you are on the money....just get 2 packs like the one you got for your caldera. They will add a tad more power over the stock lipos...and almost double the runtime.