Well you were right on the severum belonging with your fish. Although they are docile and if you have the wrong aggressive JD then you may have a problem. I'd just watch the fish and if there are problems I've solved them by singling out the problem fish putting him in a bucket for 24 hours no stimuli. Meaning no food no light but yes oxygen of course. Rearrange the tank before introducing ANY cichlid into your tank. They are so dang territorial you have to or there will be fights even a compatibility chart won't predict. As far as overstocked far from it I'd say. I'm not going to google all your fish and see the full grown size I'll leave that up to you or someone that has that time on this site.

The one inch per gallon rule isn't so much a rule as it is a guideline, and it doesn't work so well with bigger fish. Would you put a twelve inch oscar in a 12 gallon tank? No.

According to the inch per gallon rule, you are right at your limit with the sevrum included. But personally, i think you'll be over stocked. Even a 55 gallon is pretty cramped for a twelve inch fish (ie. the sevrum) in my opinion.

Once the Jacks start to try to spawn they will rip apart most of the other fish in a 55 gallon tank. Id look at getting a 90galloon because of the depth of the tank. 48x18x24 (18 in deep) They need the space to grow. Green severums can grow up to 10". As for now i think you will be fine but every fish is different and you never know.