I'm very partial to darts, so maybe I'm not the one to ask, but here's what I'd do. I'd use the new 10 for a gecko and then when you get the 20, get something other than auratus for this, like 3-4 D.leucomelas, or 3-4 P. terribilis and put in a waterway for them. Both are nice, bold frogs. The mint green terribilis are much more striking in color than any photo, because the photos don't capture their iridescence. Both are bold species and usually always in sight. The leucs climb more than the P. terribilis, but both are fun to watch. My terribilis definitely know who food god is, and one in particular tries to leap up straight into my face whenever it sees me open the top. Of course, he never makes it, but he always tries. All beg by looking straight up at you, which is charming. Both are affordable, and the leucs have a nice call if there is a male or two in the little group. I've had only one problem with the terribilis, and that was one summer when the tank got too hot for them in spite of a cooler waterway and falls. They are apparently somewhat more sensitive to heat than the other frogs I have. Although I haven't tried the Epipediobates tricolor yet, I understand that they are also bold, pleasant frogs that get along in groups. I've been unable to find a source for them at the right times. While there are others I would recommend, such as P. vittatus, these are somewhat more pricey, as are imitators, and azureus, which really need to be in even more expensive mated pairs for best results, and need a lot of floor space.
As far as the 29 gallon for the crested geckos, I think a water way and a naturally planted vivarium would certainly work well with them. It should add just enough humidity because I assume your top will be a screen vs glass.
As far as a silicone background, I'm not sure what you mean. I've tried the Great Stuff with coco fiber on silicone, used silicone to secure other back grounds, but never straight silicone. In any event, I should think you could do the same kind of background for a crested gecko as for dart frogs, except you'd probably need tougher plants over all, and would be more concerned about any poisonous qualities the plants might have if ingested, than with darts, that merely flick their tongues at moving insects, quite accurately, and never seem to accidentally ingest anything but what they're aiming for. I have no personal experience with the crested geckos at all, have only read some about their requirements, so this is just a guess. The only reptiles I have now or have kept in the past were all of the desert variety. I've had a couple of Leopard Geckos for a few years, but that's about my only gecko experience, and of course they are in a desert type tank with water bowl, feed bowl, sand and a layered "cave" viewable from the front. Otherwise, I'd probably never even get to see them sleeping all day.

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Patty
Pahsimeroi, Idaho
D. auratus blue
D. galactonotus pumpkin orange splash back
D. imitator
D. leucomelas
D. pumilio Bastimentos
D. fantasticus
P. terribilis mint and organe
D. reticulatus
D. castaneoticus
D. azureus
P vittatus
P. lugubris