You got plenty of great replies and advice to sift through. I just want to add some safe choices for isopods. These are a mix of non native European and US natives that so far after using them myself have not bothered my corns. I had a few retired corns in planted bioactives with a cool more moist side, where a hide and water dish is, and a warm dryer side where the heat pad, alternate hide, and lights are. The isopods will go to the area that has the best humidity for them as needed between watering plants.
Armadillidium vulgare - likes slightly more moist habitats where wild garters and black rats in my area live. I prefer these they are very hardy in most temperatures. Just don’t put them in tropical set ups. I wish more people kept cultures of these and bred them more for colors and such.
Armadillidium nasatum - a non native from Europe which lives in the same area as Armadillidium vulgare. I caught these sharing the same places and have both in my bioactives.
Porcellio spinicornis - another non native from Europe which again can be found alongside the previous two I mentioned.
Porcellionides pruinosus - the last non native I’ve had which can be found in my area, but I have not yet seen these in the wild here. This is my least liked of isopods. I just couldn’t keep these alive in my setups when I bought cultures to seed my bioactives that should have been good set ups for them. They slowly dwindled until they died out. Other people love these though and they are safe around snakes like the others on my list.
I also have tropical dwarf white isopods Trichorhina tomentosa. I use these in my very warm humid places like my micro gecko enclosure. However they won’t do well in a set up for temperate species like your ratsnake cross.
The isopods I have are mostly slow to medium growing, and breed pretty readily. A few species die after breeding so you may see the big ones disappear but don’t worry you probably have a bunch of small isopod babies in the tank. I would suggest dried hardwood leaves oak works great. Don’t use any softwood species, and no cedar shavings either. Some places sell treated leaves so you won’t get any pests or parasites. Though you can collect your own and heat treat it yourself. Just don’t burn your house down. I like to have bark, sticks, or even a log in the tank too all heat treated of course. The wood and leaves help feed you springtails and isopods both. I supplement my cultures with a few sprinkled cat food pellets from time to time and sliced carrot or potato. Just make sure to take out potatoes or carrots who seem to be growing lol.
I also have non native European red slugs in my set ups. They were an experiment I tried to see if my garter might eat one. They can eat them in the wild I’ve seen young ones go for them. However she left it alone and the slug proceeded to race around the tank and immediately eat the bits of snake shed. I was surprised how fast it ate the shed pieces. That slug had young in the tank and now they are just there. Lol I’m highly tempted to stick some in my corn tanks too. Now that I know they are invasive I won’t put them back outside again but they are everywhere nobody is getting rid of them.
Lastly I have red wriggler worms, Narceus americanus the North American giant millipedes, the silver springtails which i’m unsure whether they are Lepidocyrtus Sp. or the native NY Hypogastrura sp., giant white springtails Folsomia candida which are getting out competed by the silver ones, and the small Lithobius forficatus centipede. Be careful with centipedes even if this is a small species I would only use it with a subadult and larger colubrid. I haven’t had a problem but I would be worried about them potentially nibbling a baby snake. It’s not like an Asian giant centipede though these are much smaller. You definitely don’t need any centipedes I just have some local fauna set ups here. Millipedes are great though.
With all the stuff in the tank I don’t like to feed my snakes in the bioactives. I’m too worried about impaction from them eating something on accident while swallowing a mouse. I feed in separate feeding tubs. This is debatable to some but it works for me.
Edits: fixed spelling errors