Nitrates at 50 is debatable I have read it to death
yes, it does depend on the type of fish. you' ll notice that my formerly overstocked tanks had very hardy fish in them that live forever( the redhooks in the bottom I had for 11 years when I sold them)
but try keeping discus at 50 mg/L nitrate and you will have black, stressed discus..seen that one many times with customers bringing in water samples from stressed discus tanks.
so, yes, nitrate levels and sensitivity vary..
barbs are generally tolerant as are most characins.
certain cichlids, not so much.
some oddballs are very tough (eg gars) some are not (eg rays)