Tobi,
I have kept 20-25 Adults in a 125 gal tank. They were fine with good water changes and filtration but I didnt like it. They loose the ability to interact. To take up territory and spar and display. I much better like 10 -13 Adults per 125 gal. Your 50 fish per 740 gals falls in that range roughly.
I would not mix the blues, greens and heckels. I have many times and they do ok but they always seem to do better separate in the long run.
hth,
Al