In my experience you’ll have the same issues mixing red, purple, exquisite, or helfrichi that you would mixing conspecifics. I like firefish a lot and I once bought many of them because I had frequently seen them grouped together in stores - the group of reds dwindled until settling on one male and one smaller red that is only sort of tolerated by the larger. The exquisite went pretty far out it’s way in a 180 to kill the helfrichi. Lesson learned. No more mixing of firefish. They actually seemed fine at first and got along for a few weeks or a few months but that didn’t last but eventually there was a lot of fighting. One firefish per system unless sold as a bonded pair is the way to go.
I like the groups of zebra darts and they’re easy to keep in a group.