1. If you can get the spur and pinions to mesh, no reason not to try. Smaller pinions will be harsher on the spur though, as there is less contact area so the spur may strip or wear more easily.
2. Definately- look at the rc-monster heatsink clamps, or a hvmaxx motor heatsink.
3. Well, I just realized you are using a 20XL? motor- thats way too slow/ low kv for 6s lipo- you are also gearing it really tall, which is putting too much strain on the motor and heating it up so much; 190*F is much too hot, and will demagnetize the rotor, leading to a cooked esc and lipos wituhin the next few runs.
My suggestion is a different motor to better suit your voltage- I recommend either the CastleCreations 2200kv motor that normally comes with the MMM system. Its ideal for 4-6s lipo, and perfect for your pretty large truck.
Another option would be the Medusa 80mm ones, say the 1600kv version (sensible), or the 2000kv version ('fun'
)- I have the 2000kv version and it runs pretty cool on 5s lipo in my 13lb GorillaMaxx.
With the ~2000kv motors, you could gear it around 18/56 to give you ~45mph in second gear- I would lock the transmission into second unless you have all metal gears inside it, shifting on the move will eat the gears in next to no time otherwise.
You will probably have to go back to 32pitch (mod 0.8) gears, but the new emaxx 3905 ones and jato3.3 ones will fit the revo slipper perfectly.
Have a poke at the calculator to get some other options:
http://www.scriptasylum.com/rc_speed/_top_speed.html
For cheaper motor options, the Feigao 11XL or 12XL would be great on 6s lipo, but probably still run hot.