I suggest Ancistrus instead of gibby's.
a gibby can and will reach 1 foot or more adult length, despite being an easy fish, it's a big one.
few people have Malawi-tanks that are big enough to keep a 1 foot gibby
properly
ancistrus (the common brown variety) reaches 4" max or so. quite a difference.
Any medium small Tanganyikan catfish can be added in a Malawi tank, but make sure the other fish aren't overly nasty.
if you keep for example Melanochromis spp, Pseudotropheus elongatus or P lombardoi; make sure the catfishes have separate hiding places and are introduced at night after a good feed.
very sturdy Tanganyikan cats are Lophiobagrus cyclurus. due to their digging behaviour, toxic skinmucus and sheer toughness, few fish bother with them.
if you have gentle Malawians, like Aulonocara, Placidochromis and Labidochromis caeruleus; you can even add Phyllonemus typus or Lophiobagrus brevispinis.
Synodontis of course usually do fine. Due to S njassee often being very hard to get if anywhere, lots of Malawian fishkeepers here add Tanganyikan Syno's. with them, it's just tanksize and taste (and money!

) that will make up wich species.
Synodontis multipunctatus and S petricola (and related cf, aff. and subspecies) do fine, but they're groupers and should not be kept single. synodontis polli is also excellent, doesn't get too big either. S "polli White" (a non described 'new' species) is another great add-on. both like to be kept in small groups (3-6).
Synodontis dhonti, tanganicae, lacustricola and granulosus get very big (at least 1 foot or much bigger), the last one costs an arm and a leg too.......something for the huge tanklover.
S nigromaculatus reaches the same size -about 8"- and looks quite a bit like S njassee, but unlike S njassee it's a noxious, territorial beast; best kept solitaire.
1 other Tang cat shows up: Auchenoglanis occidentalis the Giraffenose. great fish, fun to keep, very peaceful (against non Giraffenoses) but unfortunately a
GREAT fish, as in HUGE.
I have one myself, and unfortunately, it'll probably be a temporary guest.....3 foot monsters being somewhat difficult to house.
Other Malawian cats are either on the "we wish" list, or on the "too big" list or both.....Leptoglanis rotundiceps, Chiloglanis neumanni, Bagrus meridionalis, Bagrus orientalis, Bathyclarias.....
The same goes for Tanganyikan cats; Chrysichthys, Malapterurus, Dinopterus, Heterobrachus......
one is on my "I WANT THIS CAT!"- list too: Chrysichthys (Gephyroglanis) sianenna, the elusive "deep sea" catfish....