ALSO...is there a reason I should be thinking now about how to divide this space into two? Like is there a reason I might be needing two pastures and divide the barn in half so two groups of goats can't access each other? All I can think of is maybe if larger goats are bullying the youngin's.
I don't see why. Especially with a tiny herd, make sure that your 2 or 3 does like each other. If they do not... then sell whichever one or ones is causing trouble. You want does that get along.
If the does get along, then the only time you will pen is right when a doe is kidding. And, keep her locked up with her kids for maybe 2 weeks... I liked locking them up so everyone can see and smell each other through a fence. I didn't want to cause issues when they were all joined back up together. It is nice to keep mom and kids locked together until the kids are a bit sturdier, and you know that the kids are nursing well and mom and kids are properly bonded. And also that mom is properly taking care of the kids.
As I said before my baby sis uses little pens for kidding. But... some of her does have low mothering instincts, and need to be right up on the kids for a few days to get everything to work as it should.
Eventhough I locked up my does after kidding, I NEVER locked them up before they kidded... when they were in labor, or if I missed that, after kids showed up, I would lock them in their own pen. They would get my entire goat pen, mom and kids. So, 10x20, more or less. Mom never lost her kids. I did, a couple of times, have to set up a warming box for the kids... but then, it is really cold where I live.
Also, I know you do probably get more milk if you pen up the kids away from the does at night... but I had no interest in the extra work, and I liked my herd together as much as possible, since that is warmer.
I did, with 2 does in milk, with kids kept on, get over a gallon a day (yes, yes, supposed to weigh, not measure by volume, whatever, the milk got poured into jars...). So, enough milk for our needs, and way less work than penning up kids.
It is important to have high production does. And, schedule! They get trained to let down at the milking times. If you milk a half hour off, you will get less milk.
I castrated the kids at about a month old, with a knife (my highly preferred method), so, no separation needed due to sex. And... I sold them young.
So... I am actually not sure why you have those stalls in your plan. Stalls are for race horses (in my mind) everything else is in a herd.
Stalls for goats are a temporary need.
I think my baby sis uses her tiny stalls for only three days. Since I only had the one big stall, and since my herd had a LARGE grazing area I kept mine locked up longer, 2 weeks, a month max. But that was mostly because I wanted to make sure mom wouldn't wander too far away from the kids. There ARE does that will headbutt and kill the kids of other does. But, if that happens... I would sell the problematic doe. A smoothly functioning herd is golden.
But also, my needs were low, because my herd was ONLY to produce milk for my human children. So... I never kept a buck, my max herd size was 3 does.
Breeding for show, for fancy stuff... totally different needs. I bred my does so that they would produce milk.
50 feet by 50 feet large enough for the main attached pasture (where the barn will be)?
That is a fine size for an exercise yard. It will become bare pretty soon.
BUT, a nice small yard means you can make sure the fence is excellent and keeps goats in and dogs out.
but I will be trying hard to keep that number in check. But I am not ignorant enough to say I'll "only" have 2-3 and that won't change 
Milk goats avoid wet feet....and water from sky. Yeah, true. And only milk in same order each time, each day. They're peculiar in some ways.
better to know now.
