Safe food source and soil mixture?

[bootsnewt2100]

I am about to add springtails and isopods to my Triturus marmoratus terrestrial setup, and have been previously feeding my springtails with rice grains and my isopods with fish flakes. I want to know if these foods are safe to keep using after I add the invertebrates to my tank with the newts. I'm worried about something in the fish flakes not agreeing with the newts or if the dry rice grains may cause impaction if ingested by the newts. Are these still safe to use? Any suggestions for alternatives?

I also am going to be using live plants this time and nix the fake ones. I'm considering a 50/50 mix of top soil and cocofiber, but know this mixture can often be nutrient-poor for the plants. Would you recommend a layered approach to the substrate? Like gravel, then charcoal, screen, potting soil, then 50/50 mix on top, so the plants can root in the potting soil but the newts won't come into contact with it? Also, if I used fertilizer for my plants, will a bunch of the chemicals stay in the top 50/50 layer and become a chemical hazard for the newts, or would it be minute amounts and not worth the concern? The soil will obviously stay hydrated, so I'd be worried about it spreading easily amongst the soil.