Strictly speaking there aren’t any suckermouthed catfish in Malawi cichlid habitats. so I would leave them out. There is an African species called Chiloglanis, although this is a river fish, not a deep lake fish, and it’s not easy to keep by any means. Don’t opt for them.
That leaves you with South America and Asia. Any wild caught L-number will be unsuitable because of their preference for soft acidic water and once the algae has gone you won’t be able to feed them. Your Africans will eat any algae wafer you drop in and the plecs are likely to starve.
From Asia there are the hillstream loaches — again specialist stream fish and sensitive — Garra spp., which are tougher but will be outcompeted and are not all biotope correct. There is one species though, Gyrinocheilus aymonieri, which is the bane of many tropical fishkeepers’ lives. It is a good algae eater but becomes territorial as it grows, much like Malawi cichlids, so is often returned to the shop for being a nuisance.
If you could get a 10–15cm/4–6in specimen which has been returned to a shop it would be tough enough to put up with your cichlids, carve out a living eating algae and give as good as it gets in terms of squabbles.
There is a tank bred suckermouth option, and that is the generic bristlenose catfish — the type which you see regularly for sale as small juveniles. My friend added some to his large mbuna tank 11 years ago and they are still there to this day. They are now large adults and in good condition, although the lack of suitable Malawi cichlid proof caves means that he has never had fry from what are usually very prolific breeders. That, or the hard water is preventing them
So if you have to, tank bred bristlenoses or adolescent to adult Gyrinocheilus are your best options, although it will be a compromise in terms of water hardness and feeding any extra foods will prove difficult.
Some Malawi cichlids are better grazers than others. The best are probably Labeotropheus, followed by Petrotilapia and Tropheops, although no cichlid will remove the green from your front glass. Use an algae magnet or scouring pad for that.