I might have a P. typus myself, which I bought unlabelled from Aquajardin in late November '09. I really wish I had taken some photos now before releasing it after the acclimitisation process, as this lovely fish is rather shy and will only come out of its cave when the lights are off plus there is no movement outside the Aqua One 620T tank.
My mystery catfish is currently sharing the 620T with...
2x Pantodon buchholzi
1x Hypoptopoma sp.2
4x Synodontis nigriventris
9x Pangio doriae
12+ Tylomelania sp. snails
The above mix is a little different, but like your tank it soes suffer from "lifeless syndrome" in the daytime! I was originally thinking of adding either a group of P. buffei catfish, but now I'm not so sure the tank dimensions would be fair on them.
Like you, I was concerned about whether the catfish needs some conspecifics, but until I discover which Phyllonemus or Lophiobagrus species I have... Have a check at tropicalfishfinder.co.uk, as I think some little species of Tanganyikan catfish are available through their linked stores ("Tropical fish 2 your door") at £25 each!

Dreaming of a full-on 5x2x2 Zaire River rapids biotope...