i have a breeding trio with the male now about 10".i think they are great fish.
ive found that they will do ok in most water except really soft. however i keep mine in very hard water which is a nightmare as my local water is soft. i add a good powder buffer with the same mineral content as found in lake tanganyika with each water change. i also have crushed coral in my external filter which helps keep it topped up. ocean or tuffa rock which i noticed in your pic will also help keep it topped up, i used to have mostly ocean rock in the aquarium but now i have very large round and smooth cobbles.
my water is 79f with ph of 8.2.
as for tank size 600l should be fine i keep my three in 450 litres. alot of books state that to keep a group of say ten adults you need a huge aquarium but i know people who have had great success in smaller aquariums. one friend has one large male at 11" and six females at 8" in a 300 litre tank and are always breeding and look stunning. they are much lazyer fish than oscars when they get bigger so my opinion is that a 12" front wouldn`t need as much space as a 12" oscar. i do 25% water change every week and 50% every month, for there size they are much cleaner eaters than oscars.
i found a fine sand substrate is best as they somtimes like to dig. and provide plenty of caves.
food wise i never feed mine pellets or dry foods of any kind thats not to say you cant. mine love earthworms bloodworms krill cooked prawns and cockles. mine also will take thawed frozen peas or spinach which they have once a week. mine never took to dry foods but ive heard to much dry food can cause problems.
you say you have 8 fronts at 2", unlike oscars they dont grow as fast some only reach 4" in the first year.
and also unlike oscars they wont pair up as oscars. in the end you`ll end up with one male who is dominant and will try and breed with all the females, at that stage you may need to get rid of any non dominant males as they may take a beating this is not alwats the case though.
good luck with them, they are great fish really stunning when you have a large male
