Here’s the IFIXIT guide you’ll need to follow to swap out your drives: Mac mini Mid 2010 Hard Drive Replacement. At Step 21 you’ll need to take the thermal sensor off, use extreme care! As its very easy to break. Transfer it to your SSD in the same location. Do take the time to read some of the comments before you start.
As for your choice of SSD’s: The Samsung 860 EVO line is a great drive. In truth it runs at all of the SATA speeds: SATA I (1.5 Gb/s), SATA II (3.0 Gb/s) & SATA III (6.0 Gb/s) we call these drives ‘auto sense’ as it will match up with your systems SATA Port I/O speed. Not all drives do this! So you picked a winner here as your system is only SATA II (3.0 Gb/s).
I’m not sure I understand you listing out the different drive sizes its upto you what your needs are. Depending on which version of system (2010 Mac Mini’s (Macmini4,1) you do have a second choice. As an option you can add in a second drive using this kit: Installing Mac mini Mid 2010 Dual Drive replacing your optical drive. Or, if you have the server version then this kit: Mac mini Dual Drive Kit
That way you can keep your current drive and then after installing your MacOS onto the SSD as well as your User accounts & Apps switch the boot drive over to your new SSD. Leaving your HD as the data drive for your stuff. That way you can reduce the size of the SSD you need.