I'm presuming that these are some species of mastacembelid, such as the recently described
Mastacembelus polli; in other words, a spiny eel rather than a true eel. That species at least is about 10 cm when fully grown.
There have been numerous posts here about spiny eels, and while I like these fish a lot, they can be tricky. You should probably review some of those older posts. But basically, they are nocturnal, prone to escaping from aquaria, slightly difficult to feed (won't eat flake/pellets), and like to burrow. I don't have any experience of African spiny eels, but with the smaller Asian species, silica sand is absolutely essential. Kept in gravel-bottomed tanks, sooner or later they scratch their delicate skins while digging, and an infection sets in. Given yours is a Tanganyikan species, coral sand would probably be an acceptable alternative.
I have kept larger spiny eels (M. armatus, for example) with peaceful cichlids without problems, but as with any bottom-dwelling fish, they are likely to be viewed as potential threats by territorial cichlids. I believe I am correct in saying spiny eels in the African lakes do feed on baby fish. Bear that in mind, and it would probably be wise to provide lots of caves and cover for all concerned so as to minimise problems.
Cheers,
Neale
