Hi There,
I was born and brought up in Malawi and Zambia, so can offer a little advice.
First up, fully agree with Hannes101's recommendation that you join one of the local angling clubs - if you live in blantyre then this will be easy.. You will get a great deal of info from fellow fisherman, and also have an opportunity to fish in the Lake Malawi comps which are a fairly regular event as far as I am aware; they also hold an international Artlure tournament i competed in a few years ago - various teams, including from SA and the region.
Trout wise - there are wild self-sustaining populations of rainbow trout up Zomba plateau - or perhaps I should say there used to be - I used to fish there a fair amount as a kid (not too long ago!) before the creation of a dam on the Mulunguzi river - up to this point the fishing was decent - normally easily picked up 10 plus fish a day - but small, free rising rainbows. There is also a hatchery there, but I didn't get the impression they did much re-stocking of the waters at all! The creation of the dam did lead to a big decrease in the trout in the streams - poaching/fishing by the large work force.. I dont know what the situation is now - worth an enquiry. Chagwa dam also used to offer decent fishing for largemouth bass (not huge, but plentiful) and redbreast tilapia - BUT, this was in the past - i suspect population pressure and the lack of real regulation has put paid to this small dam..sadly..
There were also small wild brownies up Mount Mulanje - worth an enquiry again, as not sure the status there – would guess they have also been hit hard by a growing population..
The best bet trout fishing wise is Nyika Plateau - dams and streams - rainbows, and as far as I am aware still offers a decent fly fishing venue for trout in Malawi.
Around Blantyre / Limbe - head up to Chyolo/tea estates area - there used to be decent bass fishing here - although i know a fair few of the dams are on private estates. You could often take a brace of bass - 2-6 pounds average, when I fished there - again a while back now.
Other option (excluding the lake) is the Lower Shire / Sucoma area - same species as the Zambezi (below Kapachira Falls) - so you could try your hand at tiger on the fly perhaps..!
Hope this is of some help, RF.