EBJD - Not much experience with these, hopefully you have a docile one
Severum - If you get a proven pair, be ready to see the rest of the fish in 1/3 of the tank
Geophagus altifrons - These are a great mix with Sevs, Keep at least 5 of them if you can
Dithers (such as silver dollars, etc.) - I would look at a school of tetras more than silver dollars. The school will keep shape with big fish like you propose above. They may get picked off here and there, but you can replace as needed. I would look at something like Red Eye Tetra, Lemon Tetra, or something else with some body.
The only issue I see is that this tank does not have a super large foot print if you go with the 120. Get the 150 for height! Once your sevs have size and the geos have size, not to mention the ebjd, you are going to have issues with territories, my Geos and Sevs occupy different strata so the 150 would take care of this for the most part. The EBJD would Eek out a space. Out of the list I would make a tank like these
120 Gallon
3 Sevs, 5 Geos, School of Tetras
or
EBJD, 2-3 Sevs, School of Tetras
or
Nice big Sev or the EBJD, Geos, School of Tetras
150 I would try for exactly your list
Just my $0.02