Well, you have selected a species of fish that is renowned to be sensitive to water quality and conditions; and need soft acidic water, and will never be fine in hard water.
Conversely your shell dweller needs harder alkaline water, and would not be thrilled in soft acidic water.
You need to go back to HFF, select a fish that you like, and then stick with it. Shellies are really realyl cool, so if you got more of them and removed the neo*, that would solve one potential issue.
Also, if your tank isn't big enough for e. yellows, shellies are just about the only small african I can think of that would be fine in there. maybe pair of julis or something, but then again territorial issues would be your biggest problem.
If you had a larger tank you could have experimented a bit more, but given the space confines you don't have much of a choice really, and if you do add a whole load of different species in the same tank - which is fine, you need to make sure they will be compatible for temperament and water conditions.
sorry, but it doesn't sound like their fault TBH. If you, as you have indicated above like having many species in one tank, the sales person has merely done what he needed to, to make you as happy as possible. That involved selecting 2 fish that he simply may not have forseen any problems with. I don't think the leleupi - shellie agression is all that well known about, especially given that the shellies are new to the hobby here. Given light of the new information, you just might find that the shop is happy to undo any mistakes.
This is why it pays to research things for your self before going out and buying stuff. I think it's unfair to expect everyone to know absolutely everything about a million different species of fish; life is a learning curve after all.