I have kept them for a few years I currently have 10 one of which is a full grown 6-7" the others range from 2" to 5". If you buy one or two, it is not the case that they will necesarally die, but that they are naturally found in larger numbers. There was an earlier Indian expedition (few years ago) where a couple of people went in search of the denisoni barbs and only found individual specimens, but that was a mere couple of days visit and one observation, not the general rule.
As mentioned your tank is too small for full grown fish, these guys need space.