Is this true about Danios?

For a while--they won't do well kept in brackish over the long run. Many fish 'tolerate' brackish conditions, but that does not mean they should be kept in brackish tanks.

If I remeber correctly most tropical fish dont mind a touch of salt in the tank for many it does them good especially if they have a parasite or wound (as salt sterlises/cleans it)

I'm sure that's not how you meant it, Rob, but that pinch of salt doesn't qualify as brackish in even the most extreme sense of the word.

Other than livebearers, most common aquarium fish are best with either pure fresh water (especially if those fish are sensitive and scaleless, like corys) or a small bit of salt. Even most livebearers, outside of mollies, won't do well in real brackish water, from what I understand.