In mbuna/malawi tanks, a lot of people keep synodontis catfish. The problem with these is feeding as they like ematy foods which most mbuna cichlids cannot eat (as they get bloat).
Bristlenose plecs also work commons do in larger tanks as well) and are perhaps a better option because they are happy eating veggies. The downside is that they eat algae - which means your cichlids won't have access to it. Also, in the case of common plecs, driftwood is a necessary part of their diet - not a good thing when you want your water to be hard and alkaline. You also must keep in mind that plecs are territorial so, while the small bristlenoses can be kept in pairs or medium-sized groups in larger tanks, in a small tank you should stick with either only one or a breeding pair. Commons, obviously, require a very large tank due to their potential size.
There are various odball fish that some people manage to keep with mbuna. Also, some have kept various kinds of larger loach (eg: clown) successfuly - though I would not reccomend this. Again, with most of these fish, you have a serious feeding issue as these need meaty foods which the cichlids can't handle.
Snails are ok as cichlid companions though they might get nipped. Shrimp will get eaten - and this is also unhealthy for most mbuna because of their herbivorous nature.
There are, perhaps, a few schooling species that'd work with the more peaceful african cichlids. But you need to be extremely careful when choosing.
IMO, it's best to stick with cichlids and aim for variety through the cichlids you own - rather than through the addition of entirely different species.
