A Common Pleco will adapt to living in a Malawi biotope, as long as you have sufficient caves for it to hide in. The common ones are as tough as anything you'll find, have hard armour and can certainly fight back if they need to, so I don't see any problem with that. There are several other plecos that will go well in a Malawi biotope, but I wouldn't get anything EXPENSIVE, as they tend to be expensive mostly because they are harder to keep - and less good at adapting to different conditions than their natural habitat, which is quite different from the ideal Malawi setup.
You can have barbs from Lake Malawi (I don't know how easy these are to find, as I don't keep L Malawi species myself):
Barbus innocens Inconspicious barb 8 TL
Barbus macrotaenia Broadband barb 5 TL
[There are others, but they grow quite big, and althoug they wouldn't outgrow your tank, they would take up a lot of your tank-space and not really work well as "dither fish", more like "scare them in fish"...]
There are a few Synodontis species from Lake Tanganyika (and S. njassae that come from Lake Malawi). They all like the water conditions of a Malawi setup, and they are well adapted to the agressiveness of the local Cichlid population, so can keep up with that. I'd probably go for Synodontis petricola, which is one of the easier to get hold of, and they don't cuckoo spawn like S. multipunctatis - they lay their eggs together with spawning mouthbrooding cichlids, and the eggs get collected together with cichlid eggs by the cichlid, and when the eggs hatch they eat the baby cichlid(eggs). Not a good idea if you plan on breeding your Malawians.
Breaking the biotope, but keeping with the water conditions, and relatively easy to get hold of, would be a shoal of some sort of rainbowfish. They like hard/alkaline water (generally - check individual species before buying tho'). There are a few dozen different species. I've got some Bosemans rainbows in my big community tank, and they are fascinating to watch, and they spawn quite regularly (although I've only ever seen one fry, currently at around 2" TL).
Here's a link to Fishbase's list of species in Malawi: http://www.fishbase.org/TrophicEco/Fish ... ?e_code=12
[I hope this link works!]