I have a full grown adult male JD with two semi-Juve sevs (~3-3.5") right now. They get along fine and with you interested in a EBJD, its should work fine as the EB's are usually less aggressive than their common species cousins. As Ripley said, maybe they will be good for a month or so... but i would def upgrade to atleast a 90G in the next month or two. The JD can reach 7-8" but the sev can get even bigger than that and both could be territorial. Plus, with spending the extra money on the EBJD (usually $20-25 per specimen compared to $2-3 for a normal JD) I wouldnt want the sev to turn around one day and kill the JD cause there wasn't enough space and territory in the tank.
If you do try to introduce the EBJD, make sure it is of roughly equal size and you should change around the decor of the tank and do a good sized wc (~40-50%). This will make things new for both fish and give them even footings to claim territory.
If aggression occurs still, which it will... they are cichlids and will have atleast some minor squabbles, and it seems like its getting a little out of hand, do not hesitate to remove the EBJD. Have a second tank or a aggreement with the lfs to take it back or else you will end up with a dead fish...
Ox