Complete growing noob could use some advice, and some questions answered.

Hi, I'm growing for the first time and have run into some problems, plus I have some general questions.

First, in case it matters, I'm growing 5 Alive and Star Killer from seeds. I admittedly, started late, partly due to it being a cold spring, and being busy dealing with the pandemic, so I expect that I'll end up with smaller plants.

All that being said, I've run into a problem or 2.

First problem, my plants got hit with Spider Mites. I first used a home solution I found online of mostly bottled spring water, a bit of gentle soap, and a bit of vegetable oil. I now only can see dark specs on the bottoms of 1 of my plants, and I destroyed all the webbing to see if any new webbing is made today to know if the mites are alive or dead. I later went out and bought some "BugBgoneEco by Ortho, which it says is a medicated soap. I don't know how often I can spray for mites without hurting my plants. Second, I don't know how long it takes for the white speckled spots on the leaves to vanish.

2nd, I have a couple yellowing leaves and 1 that looks worse than that, and I wasn't sure if it was from over watering or the mites. I have a general "soil moisture" meter but I don't know what these strains prefer.
Right now they are in pots, but I'll have to plant them outdoors soon, but for now, since they are small, I didn't know if a small grow light for 24h light would be beneficial or harmful to them at this stage.
I also don't know if I should prune the one leaf that looks a lot worse for ware then the others.

Lastly, but probably most critically, I didn't know what sort of nutrients I should be putting in the soil (plane potting soil) at their current growth stage.

Some are growing faster than others, and I'm not entirely sure why, but these pots are a size up from the ones I started with. One issue I am having is a few of them seem to be just barely strong enough for their height, and other then supporting them with a stick, I wasn't sure what else I could do.

Hopefully it will let me upload a coupe photos. Sorry the one isn't very good, but you'll get the idea.

Thanks!