Hi Sandy, it depends to some extent on the kind of salamander. What kind is it, and how large is the tank?
I would not recommend peat moss, as it can be too acidic. With other kinds of moss (especially the non-live kinds), you also have to be careful not to use anything acidic. Top soil is excellent. Coconut fiber is OK, but I don't like it all by itself. It's somewhat abrasive and it doesn't do a good job of breaking down waste (meaning you have to clean more carefully and are more likely to have problems with mold). If price is not an issue, I'd recommend the "forest bedding" kinds of products. As a cheaper alternative, I'd recommend just top soil, or mixing top soil with some coconut fiber. The coconut fiber improves the moisture retention of the soil.
Regarding plants, it kind of depends on whether your salamander is going to dig them up or not. It is safest to start with cuttings, as the soil you buy with commercially-planted plants may contain all sorts of fertilizers, pesticides, etc.