Sounds like you did your research. one of the owners who keep multiple fires together.. Treat them as you would cichlids. when you introduce the new ones. do it together. and mix the decor up and add more hide spots and you shouldn't have much of an issue. Get the new ones as big as possible. and be wary particularly if they are straight from importation. get them eating asap, and do not try and starve them onto foods. Let them settle in before trying to get them onto the foods you want. I suggest access to live worms, and/or ghost shrimp. try frozen krill ect first but don't be surprized if they refuse anything but nightcrawlers/earthworms. They are also very intelligent. My First eel started takeing frozen krill, and the other 2 watched her and where soon trying it themselves.
I also hand feed my 3 to make sure they all get there fair share. if I didn't my biggest one would bully the others away from the food until she couldn't cram anymore down her throat.
Very nice eel you have there!