I have a 30 gallon with 1 angel, 1 gold gourami, 8 sterba cories, 1 rubbernose pleco. I wanted to add a school of 6-8 medium sized tetras (maybe congo tetras?) to give a little bit more activity in the mid range level of my tank. Do you think this would be pushing it? My tank isn't planted but I can plant it if that would help give the fish more territory to work with.

I would go with a smaller sized tetra. Congos are on the bigger end of the tetra scale and may not be the best fit for your tank. Something like cardinal or black neon tetras would be a better fit, I feel.