inmates are fine too in their 6x12 cell.
A person could literally live in a closet if they had sufficient access to food/ water. I wouldn't recommend it and they would probably have a shorter lifespan just like when people cram certain fish into to small of a tank..... but hey its possible lol
I agree with the others, get rid of the naso it will outgrow your tank if it isn't already to big (don't know your definition of a medium sized naso). In a 120, you could do yellow/ purple/ scopus if the $ isn't a big deal and Im sure other tangs as well but those are the best in a 4' tank IMO.
As far as number 7-15 is fine (higher # = more smaller types, less larger types). I have 22 in my 180g and am considering adding a large school of chromis that will push me well into the 30s, but I have pretty good filtration and alot of extra water plumbed in too.... among other things.