I think understocking is just as much an issue with these types of cichlids though.
I am going through the same problem you are with my couple of jag fry. I am growing them out and one is clearly dominant and the other hides behind the heater all day. I guess they aren't fry any longer and i should probably move one to my 75 soon, but these cichlids need an aggression balance. It might not hurt it to add more fish to the tank, in fact, if you added a con, a firemouth, and a jewel then the smaller jack dempsey would probably come out more because the aggression will be dispersed. (this is what happened when I added some juvie convicts to the juvie jag tank)
If you are truly dedicated and will get a larger tank, then you can add a few cichlids now, I really don't see anything wrong with that as long as your jack dempsey isn't like 6 inches or more.
Also these guys aren't going to grow overnight (technically yes they are growing, but they are growing an inch a month). Believe me I wish they did. I mean yeah they (jack dempseys) will grow to 8-10 inches, but thats in a matter of a year or so. Plan to seperate them when they get around 6 or so inches or get a bigger tank and put some of the more aggressive ones in there and keep the less aggressive ones in the 55.
truly jack dempseys aren't THAT aggressive (compared to some other tank busting cichlids like red devils, trimacs, texas). I keep/have kept my 6 inch jack dempsey with severums, green terror, convicts, oscar, salvini, nicaraguense, texas, pictus catfish, and plecos (not all at once but over the course of a year probably a mix of at most 5 of these in a 75 gallon at once). Never had any problems of aggression with the jack dempsey ever. But he is also not full grow yet.
If you add fish, rearrange the stuff in the tank (duh) or take the dominant JD out for a little while because he will know he is the king and push the noobs around (you probably know this already, sorry for reitterating)
good luck