definitely quarantine the leo and be very clean and careful about handling it before handling the cresteds or anything that they will come in contact with. leos can carry parasites and other diseases that cresteds are succeptible to.

ditch the heat rock and go with a uth and thermostat.

it would be best to feed crickets in addition to something else like mealworms or phoenix worms. leos do well with some variety in their diet.

once you have the leo you can determine if the tank is an appropriate size for it. a 10 gallon aquarium is adequate for a single adult.