First, may I welcome you to Tropical Fish Keeping forum. Glad you joined us.
Now to your questions. Forget the salt; no freshwater fish needs salt. Mollies can manage in brackish water, and other livebearers tolerate salt better than most other fish, but the fact remains that none of them "need" it. [The do need mineral salts, but that is not the same thing as sodium salt; mineral salts occur in medium hard to hard water.] Having said that, if your molly was in brackish water and had fry, the fry should obviously remain in that for the beginning.
To the fry tank. You won't need a hood, though a cover is useful to keep water in the tank and dust out, also a more constant temperature (less of an issue in summer of course). Keep the water level a bit lower, just in case fish decide to jump esp as they get larger. If you had plants, light would be necessary (or a window location).
Feeding livebearer fry is easy, as they can take finely ground flake foods from the start. Pinch them up in your hand. Include veggie-based flake foods like spirulina and algae and kelp to provide the vegetable green food molly require. Newly-hatched brine shrimp are fine [you're spoiling the fry so early
] but not necessary with livebearers.
For the future; fish tend to give birth in the early morning, egg layers usually spawn then too. Floating plants is the best protection; livebearer fry tend to quickly move to the surface, and with floating plants will be more protected longer. Stem plants allowed to float work fine; cabomba and similar fine-leaf plants are ideal.