If you're serious about stocking single specimins, here's some tips:
1. Try to stock them all at once or in large batches, it's very difficult to add single fish to this type of setup as they are prone to being picked on.
2. Add them all as juveniles (as young as possible). Allowing them to grow up together helps them to tolerate each other and reduce aggression.
3. Avoid similar looking species where possible like mixing different Elongatus or male Saulosi and Demasoni.
4. Don't add mild mannered species like yellow labs to an aggressive tank, in fact don't keep labs singularly, it doesn't work.
Hope that helps, i learn't a lot of that the hard way.