In a 22L, it is unwise to have that many fish- as the bio load would be too much. So I would suggest getting the other tank mates a new home- since the rummy nose would do better in schools, and the mollies would outgrow the 5 gallon if kept with the betta.
Also would not use AQ salt for nipped fins- it can hamper the growth, cause internal damage in the way of liver and kidney failure, and can cause the fish and the bacteria to become resistant to the beneficial properties of the salt.. and should only be used for specific treatments. Clean water is all you need to prevent rot from forming from damaged fins.
Melafix is not recommended to use for them.. as the tree oil ingredient can harm their labyrinth organ.
The fins will grow back with clean water, high protein diet and if you want to add something, Stress Coat promotes fin growth.
Good luck with everything.