is it possible to keep that cage humid without making it moldy?
the answers to all your questions can be found in both the caresheet and by using the search function, as far as the husbandry and breeding tips. bear in mind they must both be FULL adults -
for the acquisition, hmmm...QT is still a good idea if you're talking a pet store (and keep them away from any other herps you might have too).
I have a few concerns. one, are you prepared for them to not like each other? it's unusual, but it happens. even girls of the same size put in a big enough cage will sometimes fight and you might have a need for two cages.
also, introduction is a little delicate. it's generally considered the best thing to let the girl establish and then add the male, but you have to watch them closely for at least the first few days and make sure no one's beating anyone up.
although it's very unusual, you can find a crestie female who will have murderous intentions toward a male and will damage him. aggressive cresties can fight to the death
if you don't yet have the geckos, you might consider spending a little more time and finding a way to get them from a breeder - are you anywhere near a place with reptile shows? as compared to a pet store gecko, you can usually get prettier animals that are less likely to have health issues for about the same price, since you're not paying the petstore markup. it would also give you plenty of time to establish your terrarium. you could make a very nice planted setup in that cage with very little money and not much effort on your part.
one more thing (sorry for the novel)- when you say "medium sized chips" what do you mean? cedar in particular, but pine in general, are toxic to reptiles.
there are also several threads focusing on substrate here. I like organic potting soil, maybe a little peat moss or coco fiber for moisture, and then moss and/or cypress mulch (mulch, not bark) on the top layer.