actually i have 2 larger flowerhorns with my oscar for now, all bout the same size, they co-exist fine, but back to the point.
do NOT put anything else with the flowerhorn in your 50g, heck, i wouldnt even put the flowerhorn in the 50g, you know a full grown flowerhorn can get anywhere from 12-16" on average? the largest recorded flowerhorn on record is a Golden Base flowerhorn at 23", so size varies, but at VERY least it will get 12", in other words to large for a 50, itll be fine for now, but i can 100% guarantee it will be excessively cramped when the flowerhorn grows, you will see.
also, i assume when you say it didnt go well, he killed them, is that not a sign that he doesnt want tankmates? all the flowerhorns i own are tolerant of tankmates, obviously yours isnt, so why force it?
eventually id like to upgrade my tank to a larger as i feel it will be to small for a single flowerhorn, but it is easier said then done being 15yrs old
terribly sorry if i come off rude to you, it isnt meant to be