Anything brought from a "mixed African tank" has the potential to be a nice fish, but its purity can never be guaranteed. If you're just setting up a community tank, theres no harm in buying these fish. Its a shame the farms won't even confirm what is sent.
If you're interested in quality fish, or something with known lineage, check out some of our club sponsors. There are plenty of really nice fish out there that can still be bought for a reasonable price. The mixed tank concept is a great starting point, but eventually you may become more interested in specialized areas of the hobby.