Don't worry about MGSV or Hitman 1 or 2 playing from HDD, both should be fine.
I completed MGSV when it came out on an HDD and Hitman 1 I played on SDD but they load most of the level once, then it's like a sandbox without sub loading so won't make much difference apart from a longer level load.
Also both games less have twitch play, lots of stealth, will be fine on HDD for now.
Game saves usually always save in user data or documents on main drive even if installed on another drive. Some Steam games will save to the Steam app data on the extra drive though, still no problem to find them.
To add your HDD as secondary Steam drive:
Steam menu top left,
Select settings
Click Steam Library Folders, then Add Library Folder to browse for and add the HDD as a secondary Steam drive.
I completed MGSV when it came out on an HDD and Hitman 1 I played on SDD but they load most of the level once, then it's like a sandbox without sub loading so won't make much difference apart from a longer level load.
Also both games less have twitch play, lots of stealth, will be fine on HDD for now.
Game saves usually always save in user data or documents on main drive even if installed on another drive. Some Steam games will save to the Steam app data on the extra drive though, still no problem to find them.
To add your HDD as secondary Steam drive:
Steam menu top left,
Select settings
Click Steam Library Folders, then Add Library Folder to browse for and add the HDD as a secondary Steam drive.