Update: I moved the severum to a 40b with 3 young bichirs. They've been roommates before, although just briefly (2-3 weeks). Unfortunately I don't have a hospital tank at the moment (thanks to the aforementioned unexpected Cyrotocara moorii + ??? babies).
JD and SDs are still together in the 75. I still hadn't seen the JD harassing the severum, but he's been getting meaner with the SDs so I wouldn't be surprised if he was bullying the sev too when I'm not around to see it.
It hasn't even been an hour but severum seems to be doing ok so far. He even tried to eat something as soon as I dumped him in the 40b, but it was bichir poop so of course he spit it out. Will see how tomorrow's feedings go.
(Just FYI, the endgame for all of this is a 135g (not set up yet - I've got the tank but need a stand and sump) that I was hoping would be home to the severum, SDs, pike, and possibly JD and some Paratilapia polleni.)