Hi Carl,
If you want a naturalistic set-up, and are concerned about the soil turning sour, use a bag of soil-based potting compost..."John Innes No.3" gives good results.
Stack pieces of cork bark and broken clay flower pots for hides, instead of using leaves. The fire sals can then choose where they feel comfortable.
The substrate should be damp at all times, ferns do well in this type of vivarium as they don't need strong light, and spraying them will keep the humidity high.
If feeding earthworms to your salamanders, use a pair of tweezers to hand feed them, so that you know how much they have eaten.