mega-thanks to Bromor Neckbeard for taking the time to address all those little things. you make the world better for newbs everywhere.
I'm glad some of my advice helped you. Three months ago, I was you, so I know how it feels. This is by far the politest, most mature, and most helpful forum on the entire Internet, I'm just trying to fit in.
i've started a new fort anyway (why can't i have more than one fort btw?)
You can have more than one fort, you just can't have more than one in the same world. If you want to see if you're dwarf enough to handle a triple-aquifer Terrifying glacier, but you don't want to abandon your current fort that you spent sixty hours on already, it's a simple matter to create another world, then start a new fort on that world.
I've been told that this is because eventually our fortresses will have a lasting effect on the world. When you divert mighty rivers or wipe out entire civilizations, it WILL change the world to reflect this. I wanted to have my mountain fort trading obsidian and water for sand from my desert fort, but apparently that's not in the cards.
should i dig deeper to get different minerals? also, the dwarves seem to be tracking the yellow sand everywhere as they walk around on the surface.
Yes. Unless you've got an aquifer, you should be able to get stone by going deeper. Also, it's not that they track the sand around, it's that when any animal larger than a cat (except an elf) repeatedly walks over the same space, it wears away the natural vegetation to expose the bare sand underneath. However, it will grow back over a few months.
are there any npc type units that are hostile towards eachother?
Well, it depends on their relationship in the greater world, which is randomly generated. Usually, goblins are at war with everyone else, but sometimes you'll get weird scenarios like, the goblins are allied with the elves and the dwarves are at war with the humans. I know that merchants' guards pretty much kill anything (except your dwarves) on sight and military groups such as siegers or ambushers will attack other species. However, due to the random nature of world generation, there's no hard-and-fast rule that says, "goblins are always at war with humans" or anything like that.