So you cannot share things like food and water across settlements is what I'm gathering?

How do I care for settlements that cannot produce one or the other?

How much security do settlements need per Settler.. Mine always seem to be in the red..

How can I increase happiness in settlements outside having Food/Water/Defense etc...

I heard a Magazine Rack.. any other items?

As far as I can tell, all settlements should be able to produce basic food and water. Some don't have access to a river or anything so you can only use the basic pumps. Others, like the drivein, don't have a lot of dirt, but they should all have some available. And you are correct, you can not share food/water/power between settlements, they need to be local.

You are correct.

I've not yet seen a settlement without dirt, so not sure how to answer this.

It's more about food than settlers. I go with a 3:1 defense:food ratio. For every 1 food, you should have 3 defense.

I disovered two of my settlers were arguing. Turns out, one of them didn't have any assigned tasks, so I made them tend to crops.
The rating went up when I did this. Could be a coincidence, though, but won't hurt to try.

I use the drive in to attract new settlers then send them to my bigger settlements (the ones with more access to dirt and the like)

Don't forget to salvage all the nuclear waste drums in the central pool of the drive-in. That will remove all the rads and give you access to the (sort of) clean water pond.

Yes. I just did this last night. I was shocked it let me get away with that and that that pool of water is an actual water supply and not some huge puddle of waste. Hey, purified water for all!

I know, would you really drink from a pool that was littered with radioactive waste just a few minutes (or even days) ago? Yetch!

Not quite correct. Food and water are shared once you set up a supply line. Only between the connected settlements, of course. Also, junk is shared. Weapons and armor inventory is not.