Texas cichlids are usually kept by themselves after they reach maturity as they get super agressive. This is not based on expirence, but what I have read over and over again on the internet.
No offence Betta Boy but yes seemingly you have no experience with any of these cichlids as from what i've seen every post of yours in the cichlid thread has been one dimensional.
Yes in your average 75-100G there would be little chance of success mixing these three cichlids, however this isn't your average tank and with a footprint of 7foot by 4foot there is more than enough room for these three to cohabit successfully,
You could keep a jack depmsey, or even a texas, in a 7 footer, but dithers would be limited because they are so agressive.
That statement is truly false, with your average JD or Tex in this size tank, you could go for even smaller Dithers like Giant Danios or Rainbows, Even the most aggressive Texas is going to quickly going to give up trying to catch a quick agile fish like a Giant Danio that poses no threat to the texas.
I wouldn't house convicts with much either, as they breed OFTEN, and are super agressive during breeding.
Again your not taking into consideration the circumstances, in a 7x4 I would be happy to add a tex, JD, O and even a couple pairs of Convicts with Dithers, Obviously along with a big group of hardy catfish like striped raphaels which would help in keep down the number of fry,
I have in the past kept a breeding pair of convicts in a 240L with a number of other fish including large tetras, swordtails, kribensis and a blue acara, with no casualties,
All it takes is a little common sense with regards to the layout of the tank and stocking
And Lastly if they want Mbuna i don't think they would be keeping New Worlds