I have heard that a specific type of SSD is required if you want to use an SSD in an iMac 21.5" (mid 2011). Which type of SSD would this be?
The iMac 21.5" Mid 2011 shipped with "500 GB, 1, or 2 TB (7200-rpm SATA) and/or 256 GB (SSD)" Storage and its Hard Drive Interface is "6.0 Gbps Serial ATA (SATA)". So any normal 6.0 Gbps SATA SSD should work.
As an example, from Crucial, Apple iMac (21.5 and 27-inch, Mid 2011) compatible upgrades
As you write, Apple seems to have undertaken some effort to make replacement of the internal HDD difficult, as described here:
For the main 3.5″ SATA hard drive bay in the new 2011 machines, Apple has altered the SATA power connector itself from a standard 4-wire power configuration to a 7-wire configuration. Hard drive temperature control is regulated by a combination of this cable and Apple proprietary firmware on the hard drive itself. From our testing, we’ve found that removing this drive from the system, or even from that bay itself, causes the machine’s hard drive fans to spin at maximum speed and replacing the drive with any non-Apple original drive will result in the iMac failing the Apple Hardware Test (AHT).
This site has extensive information on the issue and warns:
Ultimately, it is possible to upgrade the hard drive in the 21.5-Inch and 27-Inch Aluminum iMac models or upgrade or install a secondary SSD in the "Mid-2011" 21.5-Inch models and "Mid-2010" and "Mid-2011" 27-Inch models, but just opening the case is a challenge and it is definitely not for the faint of heart.
That it has become even more difficult to upgrade the hard drive in the "Mid-2011" and "Late 2011" iMac models with a layer of software complexity certainly is unfortunate. However, creative hackers no doubt will continue to find ways around whatever limitations are in place whether using software methods, hardware methods, or a combination of the two.
So this basically means you can use standard 3.5" SATA 3.0 or 6.0 Gbps SSD drives, but you will have to do some tinkering and possibly get a connector like this one, in order to avoid issues.