You wont need salt to keep either of those fish and the monster fish would be very displeased if you did add any salt as they are softwater fish from the Amazon.
Prehistoric monster fish ( Thalassophryne amazonica ) like most sedentry ambush hunters are really best suited to a species tank, they are difficult to ween off live foods and dont compete well with other fish for food as they will wait for food to litterally swim into their mouths and starve if it doesnt, smaller shallow tanks are better with fish like these as there is less area for their food to swim in. As far as activity goes you have probably seen rocks that are more active, they just bury themselves in sand with only their eyes showing and sit there until a tasty fish sits on their head.
The water cow is a completely different beast, it will actively hunt food at feeding time and eat more or less anything you throw at it, unfortunately this will include any smaller tankmates so no fish under 4" in the same tank. The downside is they get pretty big and they grow fast, you can expect one to be around 18" long within 2 years. They also dont play well with others of their own kind so its one per tank unless you have a couple of hundred gallons per fish.
