First of all, you must keep in mind that much of the advice you have and will receive (including mine) is opinion. Much of what has been offered to you so far has come from people reading other opinions and taking them as fact. An almost equal amount come from people's individual experiences. All I can offer are my own experiences.
I have not found that size makes any difference when purchasing an EBJD. I have heard these tales of EBJDs being weak and not making it past the 4" mark. I have only encountered this with one batch of EBJD... the only ones I bought without seeing first. They came from a reputable breeder/seller but it was apparant right away something
was wrong as all had elongated faces with the "beak" look to them. All died at about the 3" mark within days of each other.
Outside of this, I currently own four EBJDs 5"-7". Only one was purchased "large". The others were grown out. In addition, I currently have 9 EBJDs that are 1"-1.5" in a planted community tank. My adult EBJDs are housed with regular JDs for hopes of breeding. My sub adults are in a community tank with large barbs, severums, a true parrot, an oscar, etc.
All of my tanks get 50% water changes weekly except for the 125, which gets bi-weekly changes. I feed mostly Spectrum, but I also use flakes from Ken's Fish for the growouts a few times per week. I use bloodworms as a treat and never feed beefheart, simply because it is messy.
I have no ammonia, nitrite or nitrate issues in any of my tanks thanks to large filters and frequent water changes. I feed once daily and tend to skip a day or two each week. I don't worry about the Ph anymore. I make sure the Kh is at least 2 to keep the Ph stable for the week before the next change. I do not have a UV sterilizer on any tank, though I do own one just in case. My recipes for success are as follows:
125g: misc cichlids, large barbs, 2X 5" EBJDs. 2X FX5 filters, 50% wc bi-weekly
75g: 2X 6-7" EBJDs, 4 silver dollars, 4 jewels, one catfish. 1X FX5 filter, weekly 50% change
75g: 2X 3-4" EBJD, misc cichlids. 1X FX5 filter, weekly 50% wc
40B: Planted, 4 large angels, 2X 2" Keyholes, 1X 3" Curviceps, misc. tetras and corys, 9X 1-1.5" EBJD. Fluval 405 filter, weekly 50% wc.
I wish you much success in your plan and hope that you keep people posted so that we may learn from you as much as you will learn from us.