The schooling instinct of certain fish was a trait developed that has helped certain species live, breed, and survive. Schooling would be useful for many situations which include (but are not limited to) predation and feeding. These benefits are primarily useful for small fish, such as tetras, who would become easy targets for predators do to their very small size (though small species are
not the only schoolers, many large fish are also schoolers). Natural selection was not so kind to some species of fish, so in order to survive and reproduce, they evolved, and formed the trait of schooling. Whether it be to avoid predators or to feed, certain species developed the trait of schooling for a particular reason(s). Schooling gives fish the sense of security. When they are not in a school, which would only happen in captivity such as an aquarium and
not in the wild, their natural instincts are fooled around with. In this case they could become very aggressive as in the case of barbs, or become very skittish and hide as in the case of
Pygocentrus piranha species. In many fish they may even die or have their health seriously damaged when not in a school. This is why it is important to keep schooling fish, such as your danio and tetra, in schools. Sometimes, the urge of security is overwhelming and many fish will shoal with similar species as with your neons and black-neons. Scats and monos are not even in the same family, but will school together when there are not enough monos or scats, again for this sense of security.
i've been watching it for the past 4 hours and it still has not joned the neon tetra group.
has anyone has this type of behaivior? will this cause a problem?
This behavior is common when there are not enough of one species to make the fish feel secure. It is by no means ideal, but will do no harm. Bottom line, you need at
least 6 neons and 6 danios or you fish will be harmed.