Corries need a school of 6+ to be happy, they are very social fish. They also need their swimming room, a longer tank is better than a tall tank, big foot prints are the best for bottom dwellers. As for the substrate, you should have sand, as it is natural to their environment and they love sifting through it for food. Keeping rough substrate is a no go as it can harm their softer bellies and whiskers.
Betta sororities should ideally have 5+ to keep the lowest ranking fish from getting picked on by the other two. Also, a powerful filter isn't the greatest for bettas, they truly dislike strong flows. A heavily planted tank is a must for keeping several female bettas, the more plants the better. Breaking up the line of sight and providing plenty of hiding places is key to a peaceful betta sorority.
As for the tetras, what species do you planning on getting? I personally don't like keeping bettas with any other fish other than bottom dwellers (loaches, plecos, cory cats etc). A larger species of tetra probably shouldn't be kept in a 15gal tank, the only species I would get is neons, cardinals, or zebra danios, all of which are far more active than bettas and can stress them out.
What it comes down to (IMO) is either choose the bettas or the tetras. Depending on the length of the tank you should probably either stick to the pygmy corydoras or switch to kuhli loaches, keeping other species of corydoras in a smaller tank is a bit cruel, as they often live well over 8 years. I would like to add one more thing, you should try matching the species of fish you choose so it matches your water parameters out of the tap (pH, hardness, etc), and so the fish's parameters match their tank mates'.
@NickAu can shed some more light on keeping betta sororities (his tank is lovely)
