Females are usually just fine together even three of them. Most breeders actually keep harems of a male and 2 to 5 females. Just make sure the sizes of the geckos are fairly close and I like to keep an eye on them for a day or so and make sure they are friends. 40 gal size is fine for 3 Cresteds just give them a lot to climb vertically on. I wouldn't do more than three in that size cage and taller is better than wider.
Temps sound just fine but I would make sure you don't go much warmer than that. They say they can handle up to 85 but mine get weird above 82 or 83.
Humidity for me is not an exact science. I spray morning and night and spray them directly if they are about to shed or if it is hot(to give the water to drink). The books say 70 or 80 percent depending on which you read. I find it is 90 when I spray and 60 before. I have many 
Cresteds and have never had a problem.
Carpet sounds fine especially for young ones. They won't eat the substrate that way.
To do or not to do the water bowl, that is the question. It is always a controversial subject. Cresteds will definitely drink water drops off of their faces if sprayed. But what is the harm of having one besides drowning crickets and changing it. I think it also helps to increase the cage humidity. I always put one in hatchling to 2 month olds cages because they walk in it and it helps with "toe shed"
Hope this stuff helps.
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15.25.30 Cresteds
www.geckoclutch.com
