I have done this type of set up on my systems before so what its worth I will share how I installed and where applicable, why.
| First, for partitioning: | | 1. Should I even try to accept /automatic/ partitioning when the installer gets to that point?
No. In custom choose the 120 GB drive and auto choices may be fine but mine was to mount /boot, /swap, /tmp, and /. For reasons related to HDD and rpm's that was the order; for SDD not so much. The second drive I mounted on /Crypt [or some other name you want].
| 2. Is 120 GB large enough for the information on the other directories besides /home?
Yes. In my experience this has been more than enough. /home was dealt with differently and the data ended up on /Crypt.
| 3. Should I create a separate /boot partition on the smaller SSD, and if so, how large should I make it?
Yes. See 1 above. default size should be fine as it helps keep you honest and clean boot regularly.
| 4. How large should the swap partition be, and where should I put it? (That is, on the 120 GB or the 1 TB drive)?
I always went with 2x RAM. See 1 above.
| 5. In general, should I place a partition for anything other than /home on the 1 TB SSD?
This will explain how/why I put /home on the 120 [smaller drive]. Through the use of hard/soft links to folders in /Crypt I connected the data files I wanted to preserve on /Crypt. This use of links kept data writing to /Crypt and in so doing kept it separate from the OS drive. So /home/user1/Documents -->/Crypt/user1/Documents, /home/user1/Pictures --> /Crypt/user1/Pictures, etc. etc. This link was invisible to the user. The data files from software likewise can be linked, /home/user1/.thunderbird --> /Crypt/user1/.thunderbird; which was great for recovering the mail client and other softeware. This set-up was born of having put /home on /Crypt at first but if you migrated to a new distro or recovered from failure you tended to inherit artifacts which the new system choked on. This process proved to be a cleaner foundation from which to recover/reinstall. One had only reinstall a clean OS on the 120 then re-link, the data was never touched during the installation process. Proved so effective that I preferred do clean installs from OS iteration to the next as opposed to upgrading. There are some pros/cons to soft/hard links so research for the trade-offs.
| Now, as regards data migration: I have three user accounts to migrate, plus another directory on /home called "lost and found." | | 1. Should I even try to migrate "lost and found," and if so, how?
Can't give an honest answer, I never bothered.
| 2. I have at least two choices for migrating data and settings from the various user accounts--three for some of them.
Personally, I spent the $40 US on a external case for my old drive and moved piecemeal the items I wanted. When done I did a DoD wipe of the drive and reformatted for an external B/U drive.
| a. Connect the HDD to the SATA bus /after/ installing F27, and then force-copying everything out of each /home directory to its corresponding directory on the new configuration. (What command(s) would you recommend using, and with what options/switches/etc.?)
This will take with it artifacts which could cause issues IMO.
| b. Connect a large external HDD through a USB interface, transfer all the data to it before modifying the hardware, then re-transfer it to the system after installing the SSD's and F27.
Since you already have the two drives for the system an external case is a better option IMO. If you have spare hardware then you could mount the drive in a separate system and you have the beginnings of a NAS but that is another project. An external case would be the path of least resistance here IMO.
| c. Migrate the data to its "temporary refuge" over a Samba network (possibly do-able for at least one account, and that's the biggest account) and then re-migrate to the new system?
Unless you are integrating with windows, I don't see the need for Samba, Linux has several protocols to serve this capacity. My fav is sftp on the internal network as it uses the users' existing credentials.
| Which choice would you recommend?
| 3. Is it worth migrating every single hidden file or folder? Or should I select only those folders that I know contain customization, account, or similar settings, plus my saved documents/pictures/music/videos, and migrate those?
Hopefully the above answered this question. While seems a bit to do, the long term benefits proved this method was worth the trouble. Hope it helps you a bit.