With just a few tiger barbs, they can be pretty nervous and nip fins. I would get at least 7, but more is better for tiger barbs. I have 8 in my 90 gallon and they still nip a little.
I do think the minimum for the giant danio is a 4 foot tank, as they need lots of length for swimming.
Cherry barbs are good for a 20 high, and you could even do a larger group of them in there.
In the meantime, do you know your ph and gh?