Good looking tank David! and nice plecs!
Is that silica sand?
Stick with the S. leucostacta mate, I'm sure they will do something very interresting if/when they get old enough to start breeding.
I wouldn't trust them with a flok of Cardinals though. I haven't tried it but I know my altifrons snapped up quite a few neons as soon as they were big enough to fit one in their mouth.
I would however trust both G. altifrons and S. leucosticta (even full grown) with any tetras that are larger than neons/cardinals. eg. Red Phanthoms, Yellow, penguin, silvertips etc... that size and up.