To tell you the truth, I wouldn't put an eel in there at all. The eels are not fast enough to avoid the constant pestering from the Africans. Because eels are soft bodied, the bites from the Africans will do quite a bit of damage.
Add to that that 'brackish' eels are juvie marine eels that'll require increased salinity levels as they mature. Also, african rift lake salts are composed of calcium chloride. Non-African brackish eels evolved in sodium chloride based salt waters.
I concur w/ the previous comments. African rift lakes aren't brackish, but are very alkaline FW. There are some mastacembelids (aka "spiny eels") that inhabit the African rift lakes, such as Mastacembelus ellipsifer and M. moori, for example. (These are sometimes listed under the genus Afromastacembelus, but fishbase has them as Mastacembelus.)