How big is your hex? Is it the 22G to which you refer in your signature?
Personally, I'd leave off one of the gouramis and get a few more tetra, particularly if you go with cardinals. The larger the school, the happier the tetra. I can't say how the red-tailed shark would fit in.
And, you don't plan on using the saltwater bulb, right? That would have to be changed out if you're going to plant this tank.