As far as fish i would prob say that a cory cat fish or 2. They dont bother anything and make a great clean up fish (dead crix and poop). As for guppies.. they are good but dont really do anything and if you have both male and female they will breed and breed.

If your RETF do breed (which in a tank that small and not water enough... prob wont happen) They lay there eggs on leafs above water. You would take the leafs with the eggs out before they hatch and hatch them else where.
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3.4.2 eggs leopard geckos
1.2.8 eggs Pictus Geckos
2.6.1 eggs Western banded geckos
1.1.0 Desert Rosy boas
1.0.0 Nelson milk snake (het albino)
1.1.0 Mexican Black King Snake