The Dat and the sveni will make any schooling fish disappear in short time. You would need something with a high body that gets to be around 4'', which is going to be a bad idea in a 75 gal.
It may max out at 12" But by what I know they are aggressive then the normal sumatran dats that usually get very large.
You may try out giant danios imo
Definitely not discus. I would say silver dollars, or Dennison barbs or rainbow fish. There's not much schooling fish that are compatible with dats and pikes which are piscivores. This might be disliked but I would say get 4 or 5 Petco/smart peacock cichlids.
Reasons:
They get big enough not to get eaten,
they are aggressive enough to fit with the dat and pike,
they are very active,
they have great color
I use them as dithers to great success
because they are from Petco/smart, they will be adapted to neutral pHs and they will be hardier since they were raised in trash water.
edit* : But imo don't add anything because your tank is already pretty overstocked. You should only keep 1 or a pair of 12+ inch fish in a 75 gallon.