I would get the entire intended group ASAP. This is one reason why all shoaling species should always be introduced together at the same time. A group of no fewer than ten, preferably a few more (you have the space here). this is not a particularly aggressive species, but in smaller numbers they do have problems so adding them at the same time to a new tank environment is advisable.
It so happens that there is another thread on this very issue, adding more neons to an existing group of six, and some interactions. Hopefully things will work out in both cases, but the sooner you get the additional neons the better.