While some cories could tolerate the harder water, they are really mostly from softer acidic water, so not really a good fit. Synodontis petricola (lucipinnis) is a better fit, since they are from the same lake and don't get very big. Plus they are a great looking catfish. They are best in a group, like cories, if you really want to see them out and about more.
Frank Cowherd breeds them and often lists them in fish available at the meetings. I think the last time I saw him offer them they were $8 each for 2-3 inch fish. There are other synos as well, but some get quite big so watch for that and some are from rivers so not for the harder water your tropheus prefer. Synodontis multipunctatus, is also from the lake, has a similar look, gets a bit bigger and offers the opportunity to see some cool breeding behavior with tricking the tropheus into raising their young. But if your main focus is raising tropheus, maybe you don't want that.
I think you can probably add some bushy nose plecos as well, since they've pretty much been captive bred so long that they can handle most water. You would just have to keep an eye on them to make sure the Tropheus don't decide to harass them. They'll keep the glass clean, graze any algae and help eat uneaten food, but the synodontis will do a better job of sniffing out every morsal of food left over by the tropheus if that's what you're after.
