LA County FireHawks are fitted with 1,000 gallon tanks. Whether they fill them is another story---that would depend upon the elevation of the dip site and the temperature.
The comparison site you linked to does not show most of the helicopters used in firefighting. In the US, we typically use Sky Cranes, S-61's, K-max's and then Bell 212, 205, UH-1H, Long rangers, Jet Rangers and Astars. You will occasionally see some others too like the Fire Hawks.
Yes it is normal to fly through the smoke...do it all day long..
There were 38 K-max produced, of which I think 13 have been written off in crashes. It is an aerial truck and there was no need to have two seats therefore was made as a single seat only. There is a provision to strap one person on each side of the aircraft for transportation, but they are out side the cabin. If you are going to fly a K-max, you will already know how to fly and no dual required.
You may be interested in this post to understand fire, check out the whole thread too:
http://www.pprune.org/rotorheads/111...ml#post6709161