On the fish, to avoid a repeat, do not take the advice of fish store employees without confirming it. The best way here is to note down the fish you like, and post the species in this forum so we can advise. "Tetras" covers hundreds of species, some very small, most larger, some huge. Some are peaceful, some active, some nip fins, some attack fish, some live near the substrate, some near the surface, some need currents, some need quiet water, some need warmer temperatures than others...as you can see, all of these things matter. You do not want to come home with a group of fish and then have to take them back or get rid of them somehow.
Also see what cories they have, here too there are size differences that will matter in a 10g tank.
What sand are you thinking of getting?
Ah, that makes sense. Well, I will go take notes on Wednesday. I will set it up tonight (with sand as the substrate) and let it run.