Sorry, I totally forgot about this thread. I'm terrible at replying, hehe.. I'll try to comment on what I can, sorry if it's a bit scattered. I'm typing my thoughts as I scroll through.
I don't use any particular method with my tanks and certainly not the walstad method. I'm more of a 'this requires little maintenance and basic light, shove it in the tank!!' kind of person, LOL.
Black Widow Tetras come from cooler waters I believe, but not COLD water. No tetra should bother with short finned fish (female bettas or male plakats).
12 Hours of light a day is generally too much for the majority of tanks, stick to 8 hours a day.
Adding salt to an aquarium is just simulating (most) fishes natural environment by adding back electrolytes and minerals that tap water lacks. It's also a pretty good preventative of a few illnesses and will aid in wound recovery at a higher dose rate.
In case the LFS didn't tell you, you should take the rubber band off any plants you got and spread the stems apart to allow for growth and adequate light to the bottom of the plant.
If they aren't eating, don't feed them. Let them starve for a few days, it's not going to hurt them. It may also be they don't like the food you're offering, find out what the LFS was feeding them.. I've had many customers come back after switching from a 'premium' brand of food and say they won't eat the supermarket rubbish.. Well of course not. You wouldn't want Mcdonalds after eating gourmet food, would you? lol
Mollies and Platys make fine tank mates for bettas. Maybe not the longer finned ones, but they are absolutely fine for girls. From memory my veiltail male absolutely hated the mollies he was 'forced' to live with in a 4ft tank but they didn't bother him. Fish will vary.
Definitely work your way up to at least 6 Black Widows to make them happy. You can add more a week or two after you got the first lot (sorry not keeping track of dates). I would however buy them from a shop that is keeping them in tropical temperatures already.
Your pH is fine, don't mess with it.
By sponge filter drajit82 is referring to a secondary filter made of sponge that is air operated. What you're referring to is just filter sponge/foam/floss whatever the heck you want to call it. I'll make no comment other than I don't like to clutter my own tanks with extra filters unless it's required (ie heavily stocked) but it doesn't hurt to have them.
I'm not sure what these white dots you're referring to could be. Are they moving around in the water or are they on the sides of the filter casing? Worm shaped perhaps?
Salt doesn't (or shouldn't...) mess with your pH at all.