It is not that salamanders want to eat their bedding, it is just that they are messy eaters with bad aim. Often times they will "dive" for a cricket and come away with some sphagnum moss in their mouth. If you tweezer or tong feed him I don't think you have much to worry about as far as constipation goes. If you do not you may want to consider changing your bedding. Coco fiber, sold in dehydrated blocks as bed a beast and other brand names works exceptionally well and has smaller particles than sphagnum. It also is a neutral substrate as sphagnum can be quite low pH whch may cause problems for some animals.
As for the aquarium size I wouldn't want to downsize him if you can give him a larger home. Putting him on top of your vivs may increase the temperature in his cage, because heat rises, and this may be bad for your little mandarin.