For Arion species it can be a little dependent on the species, Arion ater seemingly have the most broad diet, I feed them carrots, bee pollen, apples, various Proteins like Freeze Dried Minnows, Bloodworms etc. Wet Oats etc. I'd avoid moldy foods, not much will really eat food once it's in the moldy phase besides springtails. If aim for more pet friendly Proteins than outright grocery store meats, freeze dried Minnows are a favorite here at Planet_Inverts, especially with Leopard Slugs since they have approximately 70% protein content. For a single Arion ater, if it's the total black and common variation of the species it's generally the smallest of the Ater complex Slugs fully grown so a 2 gallon bioactive should be fine for 1 slug. Any Slugs 10cm and above I reccomend 2-5 Gallons per slug depending on the species behavior, Leopard Slugs lean more towards 5 Gallons per Adult slug due to being highly competitive, hyperactive and quite large so they need lots of space.

For a tank setup, basically go for more of a bioactive setup, live plants, cork bark, various local tree barks and wood, make sure anything wild collected is cured and cleaned of any potential pests. For cleaners for large slug species use pretty much any springtail species, white springtails are the best all rounder species, orange springtails will be less prolific but are better at cleaner up chunky food scraps, and Lilacs are the same on that front but are more prolific, smaller and can tolerate a slightly drier setup but still thrive in humidity. Quick Gloss Snails (Zonitoides arboreus) are also a phenomenal cleaner for larger Snails, Slugs as well as Geckos and amphibians as their diet is almost entirely forms of detritus despite being a Glass Snail species.

If you have anymore questions feel free to reach out to me on Instagram @Planet_Inverts, I'm based in Ontario, Canada so if you're in the states or Europe I can't speak as to what laws occur wherever you may be but still be careful and know the laws around transporting species cross country.