Hi and welcome! Letting the tank run empty didn't cycle it I'm afraid. And with that many fish in a 7 gal
tank, particularly one that isn't cycled, you're seeing the toxins spike.
Tetras are typically active and need a larger tank. Guppies are rather messy and need a larger tank as well. Same with the molly, which also grows to a good size. The tank is really suitable for just the betta alone.
What test kit are you using, liquid or strips?
Have you tested your tap water for ammonia, nitrite and nitrate? If not, do so, just to see what you're starting with.
How high are the levels exactly? "sky high" can mean a number of things.
I'd keep up with the water changes of 50% at a time to try to get the numbers down; ideally you want ammonia and nitrites at <.25 and nitrates under 10.
If your 80 gal
is healthy and established (cycled) you can move some of the filter media from the 80 gal
filter to the 7 gal
to help cycle it (add bacteria).
You'll still need to remove most of those fish though as they are just going to keep contributing to the toxin levels in the tank and the tank is overstocked.
Here's a couple of guides if you haven't seen them:
I just learned about cycling but I already have fish. What now?! - Aquarium Advice
Guide to Starting a Freshwater Aquarium - Aquarium Advice