At 6" you might not want to have any water in his enclosure, they could drown and it could bring the humidity too high. Instead, just
bathe him a few times a week (1-3 I'd say) to keep him
hydrated.
For crickets, I'd buy as many as you can afford at once (be sure they are the right size) because he will go through them like crazy, a properly fed young dragon can eat all those 100 in a day (feed 2-3 times a day 10-15 minutes each time), personally I would recommend some variety though, so buy some crickets, silkworms, possibly roaches, and some butterworms/waxworms for a treat.