Since you have primarily Tetras, why not continue with more types of unique looking but suitable Tetras? I've always been partial to Golds, Penguins, Bloodfins and Phantom Tetras. You could also get some pencilfish or hatchetfish as these too have a more unique shape or personality and would go well with your small specie Tetras as will look sharp in a school so their being small fish won;t be an issue when the school presents a larger presence. You will cut down on potential aggression with this mix as well as larger species of Tetras can be or become very aggressive.
These are living animals and the expression " tie every thing together" is a little unsettling to me.
You have a very well tied together collection already as all these fish are native S. American fish belonging to the same family. There is no need to have a "showcase" fish to this collection as 1 large fish is more apt to cause chaos than peace amongst these smaller fish. Having the assortment of suitable fish , even tho same sized fish, is a showcase in itself.
Hope this helps