Yeah, the size of the fish and their temperament with each other are the key factors. A male severum can be a big, bullish fish. I had a 7" male turquoise severum spawn with a 5" female green in a 55 gallon quarantine tank once, so I left them there to hatch the fry. Things were fine until the fry were about a week old, then I came home to find the female completely shredded and missing her entire tail, which never grew back. Since then I don't keep breeding pairs in smaller tanks.
Then again, I've had gold severum pairs in the past that spawned multiple times in smaller tanks and never gave me a problem. It's just the luck of the draw, I guess.