I had a hi-fin rainbow that co-habed with a breading pair of salvinis for a few years in a 75 gal tank - the rainbow wolf was totally boring so I bought a 5" Hopla Mala that destroyed the rainbow within' 48hrs. The Salvini's were gettin' beat so I moved them.
A year later I realized the Wolf was growin' at an extermely fast rate 6-12" in 7 mo.,
12-14" the next few months so I moved him into a 120XH w/ a breeding pair of 12" Red Terrors, 10" female jag, 6" Male salvini, 5" male Firemouth and within' minutes these fish were all at the top of the tank. The wolf went after them like gangbusters.
I put him back in the 75.
If I was gonna upgrade his tank, I'd probably ad either Giant Rapheal cats or Giant Armored cats and mabye a medium to large pleco and be comfortable. Also wouldn't have any issues adding small tetras like neons. The Mala won't waste energy chasing food too small to amount to anything or bite fish that hurt but that's about all.
If you start with a large tank and a small Mala or Red cohabing would be easier. I wouldn't recommend buying adults for cohabs.