Getting them when juveniles doesn't mean that when some of them grow bigger they won't bully or eat the smaller breed fish. It's a basic instinct, not a friendship for life. They may be fine for a long time until you wake up and half your fish have disappeared. It's like putting baby cichlids with cherry shrimp and expect them to live happily forever.
This is why I recommended a spare tank available with a filter running in the main tank, you cannot guarantee any cichlids getting on because someone on a forum said they are ok when the next person will disagree. Once they are fully grown changing your mind and adding a different cichlid is asking for trouble and a youngster would be most likely bullied or eaten by the other fish. And you also state you dont even keep cichlids so why are you sticking your oar in? bandying round the forum saying suriving is not thriving when you havnt got a clue about cichlids, its people like you sticking your two penneth in when someone is asking a serious question that gets matters confused and fish end up suffering.
I am no expert on cichlids, but I am on the personalities of my own personal fish and with 16 tanks of cichlids from dwarfs to oscars, and rescuing cichlids I am pretty sure I am better advising the OP than you, so why are you having a dig at my advice!
I am not having a dig at your advise but if you felt so it is your own conscience talking. If you have to stick to what the OP has asked, then don't suggest another 16 tanks on top of his 75 gallon for trial and error as that may not be his idea of keeping fish due to the space/money/time required and even tolerance to watching fish beating the hell out of each other all of a sudden in the morning before you have to go to work. Sometimes it is better as well to learn from the mistakes of others, rather than you making those mistakes. Since you are so experienced in cichlids, can't you just point out at least two type of them that will 100% get on together in a 75 gallon tank and will not eat at any stage of their life smaller fish like barbs, corys, apistos or rams?
The point I am trying to make is you cannot state 100% that cichlids will mix even dwarf cichlids. I have experience with cichlids because I rescue them!, hence 16 tanks. I will give you an example, If you look up rainbow cichlids it states they are a peaceful cichlid, yet one that came into me is a little devil and lives with cichlids far bigger than it should be. The chocolate cichlid - google = peaceful!!. I have one here it came to me because it was brought to be put in a 2ft tank then the owner found out how big they get. It lived with my urau quite happily for 10 months, then the urau took a dislike to it, rearranging the tank didnt help so I moved the chocolate to another tank, where it lived ok for a while then it took a dislike to my blue acara (bullied fish suddenly becoming a bully). Most people will say chocolate cichlids and severums will get on ok and I have seen chocolate cichlids in tanks with severums myself, but when I tried my chocolate in with my belem severums it bullied them, no damage to the severums but I noted their behaviour change. Now the chocolate cichlid lives happily with my rotkeil severum. Now why this chocolate cichlid hated the belem severum but likes the rotkeil severum I have no real idea, it could be the rotkeil is smaller, or the chocolate cichlid feels more comfortable in this tank as its set up is slightly different, I dont know I cant ask the choccy. Now a differnt chocolate cichlid might be accepted and live with my urau without any bother - its the personality of this particular fish that the urau took offence to!
I fostered a wonderful gentle oscar for a while, I put 2" baby severums in the tank with her with no problems - I would
definately not recommend 2" fish in with an oscar, but this one was gentle and sweet and out of the other oscars I have fostered, rescued and own I have not come across another like her and most likely never will.
The blue acara I have, again a rescue ripped everything in the tank to bits, hence it ended up here, yet I have no problems with her. I could go on with a list of fish I have/had that were problems to others, yet for me they behave - not necessarily in the first tank I try them in I will admit.
I could also give you examples of people with cichlids that live happily together, yet someone else with the exact same fish have nothing but problems.
You cannot tell someone a 100% yes get eg. 3 severums a blue acara and a jack dempsey and they will get on, you can only advise getting juvinile fish so they sort out their differences while they are small and do less damage and sort out who is boss. I am not saying watch them rip each other to bits, this does not usually happen with juvi cichlids, a bit of chasing and occasional fin nipping. Cichlids have their own personality, no two fish of the same species are the same, some are bullies, some are gentle. They are totally different from keeping a tank full of tetra! I cannot keep dither fish with my cichlids I get aggression problems if I do, yet lots of cichlid keepers never have a problem with dithers in the set up. If the op wanted to put i.e. a jag and blue acara together, I would have posted no chance what so ever, but the fish the op is looking at
"should" work out fine, but then they might not.
The spare tank/container with a filter is an
emergency back up I keep a spare tank for quarantine, hospital or emergency. A quarantine tank is a must before introducing a new fish, tank wipe outs are devistating - been there lost 5 out of my 6 beautiful texas cichlids through not quarantining a fish - lesson learnt!. I would not recommend 16 tanks to anyone, its hard work, my days are filled with maintaining tanks. I am just recomending a single tank as an emergency, not a huge 75g emergency tank, just something big enough to keep a fish in for a couple of days if needed.