I started with a group of 7. Lost one to a jump, then lost the next 2 to aggression amongst themselves. After that I said screw it and tossed them in with my Brassies as it seemed they were just going to continue picking off the weakest.
Had my 4 brassies and 3 Hillari (lost another to a jump) together for a few years now. Very rarely do I see any damage on any of them. Group seems to work well. I have one Brycon amongst them.... I call them my hateful 8.
I believe them to be one of the most underrated fish out there..... Activity level is only rivaled by other Dorado. Mine have slowed in growth a bunch around the 9-10" mark.