The number one rule for a Cichlid is to protect the children, or to protect the preparations for making children. That can get twisted in a confined space, but it is their strongest drive. When it does get twisted, it's generally on us for something we set up wrongly.
That makes every individual Cichlid a time bomb of sorts. When the hormones hit, so will the drive. Sometimes they pair, and two will be happy together. Otherwise, you often end up in a situation where each Cichlid can need its own tank, unless the tank is huge. Few tanks are.
I very rarely combine Cichlid species. When I do, it's with a Plan B. If it doesn't work, then it'll call for 2 tanks. Angels are one of the hardest to predict. They do get very violent, but they don't have a traditional turf. The opposite is Malawi mbuna, where crowded habitats have made them develop a constant low grade battle lifestyle. You can crowd them. Angels in a fishtank are a problem.
There are some milder Cichlids, where only individuals sometimes get rough. Pelvicachromis kribensis, Apisto borellii, or pucallpaiensis, and some other odd Cichlids will come to understandings. I had a pair of then "Steatocranus" irvinei in a tank with Chromidotilapia guentheri, and both species raised babies together apart. There was an invisible line in the tank neither species crossed. I've only seen that once.
Hey, you can keep male bettas together in large enough tank, though they'll have shredded fins and some may be eventually killed. If one tiny thing goes wrong, it's awful what happens. The next time I'm on a forum and someone announces they've done this and it's worked for 3 weeks won't be the first. The original posters in those threads vanish pretty quickly when their fish start to vanish first.
Angels live together in groups in nature. Give me a few billion gallons and I'll make that work. It'll be lovely. In a 75 gallon, I have kept angels together for up to a year. And then, the removal to other tanks starts. It's often how you get a pair. It's also often how you get Multiple Tank Syndrome and end up with a collection of lovely tanks.
