ive seen killis, flagfish, darter, crays, puffer and such mixed in feeder tanks here. i got a very nice male zebra obliquedens that was mixed with the mixed african tank.
hard to tell.. both of my LFS are really bad about labeling tanks.. one of them has tanks set up so that jumpers always end up in the tank below them and the other... well.. the other goes through phases of "i dont give a crap" and the whole store goes to hell... it was like that today... very very disapointing..
I've seen endlers in a platy feeder tank and once I saw a Beautiful catfish (don't know what its called) in a mixed tank of africans, eating a fish 1 1/2 times it's size!