Look, if you've done the partitions, leave it as is.
It's not a problem, it's just a hassle, especially if you run out of room on your partition. Next thing you have to move folders, have softlinks to them etc etc. I was just trying to warn you (albeit a bit too late... 😅 ) to avoid them unless you're using multiple disks.
Yes, there will be one partition holding all your directories: /, /usr, /var, /tmp, /home etc.
As to swap, you would be best to also forget about a LARGE  partition for swap. [Edit] Just create a minimal one, 512MB/1GB. I will tell you why.
You're intending to build packages on it from source. This 'could' mean swap is used or not, depending on the packages etc.
So, why not use a swap file. This way you can make it, say 500MB initially. See if it fills, then you can add another. If it still is insufficient, nuke the lot and create a larger swap file and so on.
If you read here: http://www.netbsd.org/docs/misc/index.html#swap it will give you an indication of what to do.
As it says, old swap rules don't really apply as much these days because of the amount of RAM and design of the virtual memory system in kernels (especially *BSDs).
The absolute maximum I would think you would require for an 8GB RAM system is about 3GB. If you're running 8GB swap and you're filling it (or close to) then you've an urgent need to buy RAM before you kill your SSD.