Welcome to Tropical Fish Keeping forum.
You have quite a few fish in a relatively small tank, 48 litres/12 gallons. The nitrate at 40ppm is indicative of this for one thing. I would not suggest going beyond the 6 tetra and 2 mollies and shrimp.
To the guppies, a deformed spine is often genetic and it happens. The Guppy
may have brought something in with them. I won't speculate on disease issues as i do not have the knowledge.
Live plants would help, even stem plants or floating plants. Nitrates at 40ppm is considered OK by some but not by me. I recommend keeping nitrate below 20ppm, and in a balanced tank this is easily done via live plants and/or partial water changes.
Make sure a couple of the foods you feed are plant or vegetable-based for the molly. This is good for other fish too, but molly need it.