Nice one! I'm thinking it's still the wildness in him at F1, but it will be interesting to see what happens when stalsbergi have been around long enough to get more generations away from wild.
Wild rivulatus I've had were hardly mild, even at 2". Wanted to fight everything, including each other and other fish 2-3 times their size. Captive bred rivulatus I've had were good citizens, other fish couldn't intimidate them, but if no one picked a fight with them they were mostly content to mind their own business. I've also had them keep order in the tank, break up fights between other fish, etc. Had one that was buddies with a gold severum in the tank. But, like a lot of species, I've also seen the occasional tank bred rivulatus be extra belligerent.