Sometimes pairs/groups can live together in peace but usually problems starts if/when they start spawning and they tend to start it unbelieveble young! The truth is you never know, these have their own personalities but as far as I know, they are more or less agressive always when spawning and especially with fry. The more agressive pair I used to have kept my Congos at last corner living in 20 liter of water and kept the rest 430l for themselves! They had a new fry every two-three weeks, I got tired and sold them away. The other pair, wich we still have in sons 300l community tank, is more peaceful when spawning but as I told, plecos take care of it, thank God. The male was in pretty poor condition after his brothers (who was this very agressive male in Congo-tank) violent behavior and I had to rescue him quickly to another tank not to become killed. They were sold to my son as a mated pair. Blah...Both were males.
When it comes to water parameters, our tap water is poor with very high conductivity (tap 620-650uS, tank 750uS before WC, including a lot of NaCl and NaOH) but there has neverever been any problems with angels. We keep water clean, WC 40-50% weekly so maybe this helps. The (only) pair we nowadays have, is really cute and peaceful with all other fishes in the tank but all they do is pegging for food. My son, who owns the fish, loves them but I personally find them a bit boring since I like more active fish (thats why bows are my favorites!). Couple of years ago we used to have angelyoungsters growing and pairing in three tanks but I found them a bit too stressful because of this continuing agressive spawning behaviour even with one pair in tank. Sometimes, Iv been told, group three works fine, sometimes, if not spawning, they live in peace even in groups, you never know before you have your owns and see how things progress. Having wood and a lot of plants might help pairs not seeing eachothers but this testosterone monster I had, really hunt his poor brother where ever he was or was trying to hide! Even females can fight badly but this kind of behavior we never had, only males fought.
Hope this didnt scared you, buy 6-8 youngsters and see how things go. This is too many adults for 300l you are having but Im pretty sure you have to give away anyway few ones and you will end up 1-2 nice mated pair/s or a nice group of just females/ just males. Just keep eye on them and Im sure you`ll find your way to do. BTW, females can "spawn" together but they tent to be less agressive. These are true "The bold and the beautiful"-fish when it comes to their social life!