Leopard Geckos and African Fat-Tailed Geckos
Low to moderate humidity. Environment could contain many micro-niches for hosting a larger range of springtail/isopod biodiversity.
Go with the Straightforward Choice for a simple cleanup crew or the Advanced Ecosystem suggestions to take your bioactive ecosystem to the next level!
Straightforward Choice: Springtails
Entomobrya unostrigata (Cotton Springtails) - Thoroughly proven as the best true arid springtail. E. unostrigata are often unavailable online so a common secondary choice is Entomobryoides purpurascens (Wood Runner Springtails).
Advanced Ecosystem: Springtails
Mix Pseudosinella violenta, Folsomia candida, and Entomobryoides purpurascens to step it up a level and cover more environmental niches. Optionally add even more species like Entomobrya assuta var nov, Desoria trispinata, Seira dowlingi, Lepidocyrtus sp, etc. to maximize biodiversity. See the full species information list for picking additional species.
Straightforward Choice: Isopods
Porcellionides pruinosus (Powder Blues or another morph) These are highly active, fast-breeding, and effective isopods.
Advanced Ecosystem: Isopods
Mix Porcellio scaber, Porcellio dilatatus, and Porcellionides pruinosus to maximize biodiversity. See the full species information list for picking additional species.
Quantity
Quantity of springtails and isopods required for a bioactive enclosures varies based on enclosure size and the amount of time the enclosure is establishing before adding the main inhabitants. As few as 100-200 springtails and 10 isopods per species could seed a 36x18x18in enclosure if it is set up well in advance and the cleanup crew is given fish food once or twice a week to help them grow in population. For the 36x18x18in example I recommend ~500 springtails and ~50 isopods if they are needed immediately. You can get away with starting with fewer isopods or springtails in the environment and still have the main inhabitants, but it will take a while for the CuC to start performing their duties effectively.
Crucial Information
Avoid the protein-hungry isopod species Porcellio laevis as they may harm resting geckos. A leaf litter layer, and ideally also bark hides on the ground, is essential to ensure the isopods do not all get eaten by the main inhabitants. Hydration stations are important for the cleanup crew and for increasing biodiversity. A simple pile of sphagnum moss that is always kept moist under the water bowl can suffice, but if sphagnum moss hydration stations are spread throughout the enclosure in dedicated caves or underground pockets then cleanup crew activity increases, as does the ability to add more biodiversity. The water bowl itself will not suffice for a hydration station as springtails can't lay their eggs in water and isopods cannot navigate hydrating their gills in a water bowl without potentially falling in and drowning.
Additional Information
Always provide a thick layer of leaf litter for the cleanup crew to hide in and graze on. For a naturalistic look, I recommend adding crushed maple leaves on the floor with live oak and magnolia leaves on top. Fallen leaves make up a large part of the isopod diet and soft leaves like maple are their preference. Springtails will also graze on the maple leaves as they decay. Ideally, the soil in the enclosure should have some detritus (like compost) for an additional food source. If mixing up your own soil blend, add crushed leaves and white rot wood to the soil or purchase a high quality compost or topsoil to mix in. Detritus as it breaks down further is also food for your plants. Supplemental feeding with fish food is a great way to keep the cleanup crew population thriving and ready to tackle the main inhabitant's waste.