There are a couple of things that you must do in order to 100% successfully introduce new fish to a shol.
1) Acclimate the new fish to your tank as much as possible by getting them usto the ph, water quality, temperature etc.. by taking a seven gallon bucket of water and putting the fish in while you do a complete water change to the tank.
2) Change the set-up of the tank. break down the established territories so they don't fight for them.
3) Once you have done a complete water change to the tank and create new boundaries /territories have an aboundance of food ... give them more feeders than you can imagine. It is importent to keep a surplus of food in the tank for a month or so to make sure they know who the prey is.