In a tank the size you have I would go ahead and get 12 at the same time.
Of the 3 species of rummies, the one you are likely to see is Hemigrammus bleheri. aka Firehead tetra. Hemigrammus rhodostomus is rarely seen and Petitella georgia is rarer yet. Care is the same for all three.
Here is an excellent article on the three species of Rummy-nose:
http
/www.petsforum.com/characin/carey/ar..._rummynose.html
edit: I just wanted to add that these fish usually look rather plain in the store. It can take some time for the red to really get brilliant on them. The pattern on the tail will get more striking too as time goes on. A lot of tetras are like this, they don't reach the full potential until they settle in for a while but when they do they can be quite beautiful.