6x4 will be one heck of a purchase and no doubt have to be built on site.
That opens a huge set of options with regards to catfish and plecs but to be honest comes down to what you like. If you were thinking Niger cat then yes, too big, even for this tank, but many other girraffe cats would fit, other than the really large ones.
Again , many options around pims, a few hemibagrus, tig or Jurense.
would avoid the turtle as they tend to be messy and bring different challenges.
cats will really be based around “living with rays” both from a point of view of safe from the ray side and safe from the cat side.
personally I don’t do bare tanks, or ponds and both my tanks and pond have sand or gravel and decoration, plenty of wood, pipes and rocks, just cos I find it helps the fish act naturally and my role as fish keeper is to a) give them the best quality water I can and b) give them the best life I can, which to me means making it as natural as I can in a tiny (in comparison) box.