This is something you witness in pet shops that either 1) are stuck for space 2) don't know any better. It's not good to keep different species of Dart together for many reasons, firstly due to the chance of breeding but more due to the fact that many species can become aggressive towards others. Some Darts require a little more care than others, whilst some are more hardy. Some are bigger Darts whilst you can obtain smaller species, straight away there is a risk. It would be nice to have a tank full of different species, but it just can't be done with guarantees that they will all be happy or safe.