Welcome to GF! The really cool thing about a bioactive setup is that you never (!) have to change the substrate as far as I'm concerned. The whole point is to have an ecosystem set up that is largely self maintaining. There's another person on here that has done bioactive setups for leopard geckos and hopefully she'll respond. She wrote an article about it for Gecko Time here: http://www.geckotime.com/creating-a-...d-gecko-setup/
I have other geckos (fat tail and E. hardwickii) that need bottom heat where I wanted plants as well. I've finally decided that the best bet is to get succulents and put them in pots in the enclosure (I do ceramics, so I make the pots) as opposed to planting them.
Here are answers to a few of your other questions:
What are the recommend insects for people who are trying this for the first time?
Most people who do high humidity planted enclosures use isopods and springtails but the leo environment won't be humid enough for that. I have used dermastid beetles which look like tiny versions of mealworm beetles. They do tend to multiply quickly and then I end up with larva in the water dish, but they get the job done.
How many insects/species should I have in an Exo Terra 24x18x12?
Start with a few dermastid beetles and you'll have a whole lot, sooner than you can believe. Also check out "blue feigning death beetles (yes, really)"
Would a 2-5% UVB light work for the plants?
I just get a grow light from the hydroponics store and don't worry about UVB
How can I keep the humidity between 10-20%?
Use succulents in pots, take them out to water (succulents are supposed to be allowed to get pretty dry and then drenched), and don't worry if the humidity goes up some.
How can I avoid hard water stains when I spray the side in the morning to let him drink?
Use water from a britta filter and you won't get stains. It's better in general not to spray the sides and just give him a water bowl, especially since he'll probably be sleeping, not drinking, in the morning.