The problem with lions is that they eat large meals, however infrequently, but large meals mean big poop, which means you've got to either filter really well, or let the nitrates rise and not have corals and/or sensitive fish. They also are known for eating fish that can fit in their mouths, or sometimes not even fit in their mouths.
Yea the sailfin should be in a bigger tank, but you can get away with a small one, it'll take a year or so to grow, and by then you'll know whether you'll want to upgrade or get out of the hobby. That's a decision that most people face in the first year of being in the hobby.
My salfin is happy as a clam in my 120g, he's staying a medium size, not growing any larger I don't believe, because I've seen them much larger than his size. He used to be in either a 90 or a 65, don't remember what size it was, and he was actually doing really well in there as well. You can usually scrimp as long as you've got good filtration and a well established tank.
I appreciate your enthusiasm of stocking your tank up a bit more, but didn't you have a crash less than a month ago? I would suggest not adding anything for 3 or so months after a major incident. Just my advice, I know it's very conservative, so take it or leave it. Good luck though, and we're all here to help
