I own 6 Platydoras armatulus, having kept them from <1" fish all the way to nearly 12" TL monsters. They are extremely passive, except to each other. They will grunt and push each other away for food. Fortunately they lack many offensive capabilities, so this is typically a very loud display with little damage. All bark, no bite.
However, they have amazing defensive ability. I had a red wolf fish with one in a 65. The wolf fish bit the face of the catfish, which then ripped it open with the spines that run its length.
Keep them with basically whatever you want (so long as it fits into their temperature range.) However, if you are keeping small tetras that sleep on the sand they may turn into food. I wouldn't keep them with anything thinner than a giant danio once the catfish reaches full size. Also, do not keep them with small snails if you intend to keep these snails. They will rid your tank of bladder snails, trumpet snails, etc. The severum would be fine 100%, but will outgrow your 60 depending on its footprint. Pictus cats are also too active for a 60 in my opinion (again depends on footprint).
I keep mine with a JD currently with no problems.