1.) yes, you could even keep an adult pair or two females in a cage that size very comfortably.
2.) Yes, the Crested Gecko Diet by Repashy is beautiful because it is a complete diet and gives all the nutrients they need throughout their lifetime. Best of all, its got loads of flavours to try for variety. It never hurts to feed gutloaded and dusted insects once in a while as a treat, but some geckos won't touch insects while others love the hunt.
3.) UV is not necessary but a low level UVB bulb doesn't hurt at all either, especially if you have a live planted viv, the plants will sure like it! (It is also recommended for geckos that have MBD sometimes, as it can create a day/night cycle and help with their calcium absorption.)
4.) Exactly what menace said. Temps sound good, and Humidity can peak in the evenings when you mist, (they like to drink water droplets off the leaves and walls better than drinking out of a waterdish) but the cage should be allowed to dry out a bit during the day, otherwise you can get bacterial growth.
Help! Crested Geckos! 0.1.Leachie, 2.1.1 Gargs, 1.0 Chahoua, 2.1. Blue Tongue Skinks, 1.1. White Lined Geckos, 1.3. African Fat Tails, 2.2.0 Tokay Geckos, 1.0. Giant Day Gecko, 2 tarantulas