Instead of cardinals, I would go with more neons. Many species will not shoal together with other species. So, even though the two look similar, you will have two groups. And since they do look so similar, why not have a larger neon shoal? I have 10 in my 20g long. It looks amazing. A very different sight from 3 or 4 hanging out together.
But, that is all just my opinion.