Look for good size, not thin, check eyes, they seem to be prone to pop eye but they also seem to be able to cure themselves if left well alone, nice colouring so good contrasting blue and black. They seem to be pretty easy to sex, even at a young age the males have much longer fins. And so on...
As for large fish... you do have to bear in mind just how large your severum is going to get... and a Green Terror is exactly that, i'd not put one in with an EBJD because when young they are wussy! GT's are holy terrors! Holy Green Terrors in fact
Blue Acaras are nice... but the chances of finding a pure one are getting slimmer and slimmer, they are coming into shops as GT hybrids even though pure Blue Acaras are ordered. I do love them but... no way you can get a bigger tank? Even a custom taller or wider tank rather than length?
Personally, I would stick with your EBJD, Severum at a push a pair of EBJDs but not severums unless you can upgrade tank in future and a nice shoal of dither fish like Tetras, if using Rainbow fish as dithers, bear in mind they can get 10-15 each depending on species! Personally, Bleeding Heart Tetras and Bueanos Aires Tetras are my favourites once coloured up and as a large shoal, they shoal quite tightly and will look stunning without competeing with your cichlids.