First I must warn you the newer 'Thin Series' iMac's are a bear to open as you need the correct tools and follow the IFIXIT guides to the letter as any mistakes can kill your display! Which would be an expensive error.
OK, here's the details:
First if you have a Fusion Drive'd system (HD & SSD) you may want to leave things alone here. You will need to break the fusion drive set before you replace the HD with a SSD and you'll need to leave the drives independent.
If you have a HD only system then you may want to look at a SSHD to replace the HD. That way you gain the speed of the SSD the SSHD has for boot up and yet still have a traditional HD for your data.
If you have the bucks then going with a SSD is also possible replacing your HD in the HD only system. But most people find the cost of the larger SSD is prohibitive and tend not to like the system with a small SSD alone (512 or smaller). You may want to think about an external Thunderbolt RAID drive as a better solution.
As to the drive you are correct this system uses a 2.5" drive SATA drive (either HD or SSD will work here). It also has a M.2 type of blade SSD connection which on the back side of the logic board.
As to Samsung SSD's we use them and don't have any problems! You do need to use the newer cut of Yosemite or El Capitan to enable the OS TRIM services.
Follow this IFIXIT guide: iMac Intel 21.5" EMC 2544 Hard Drive Replacement