Be careful!
Installing an ssd in an 21.5" iMac is somewhat different than a 27" iMac. They use different cables and brackets. There's a good tutorial on ifixit.com.
I have the same iMac as you have and i've tried it myself.
I wanted to make my iMac ready for an ssd upgrade. I still don't have an ssd but i wanted to look inside my mac and install the cables first(i used to build pc's and i was kind of curios).
I bought a 50 cm(20") sata3 cable and a sata power splitter finding out that the sata cable is to long(you need a 30cm (12")) and power splitter not usable because its connecters aren't 90 degrees angled(i thought i read it somewhere that this would work but that was probably for the 27" iMac)
I advice you to get the iMac 21 ssd kit from ebay consisting 3 parts (power cable, sata cable and ssd brackets) or the ssd upgrade kit from ifixit with the same parts and some extra's for not much more money.
some other things:
The fans will only spin to max when you REPLACE your hard drive with another non apple drive! Not when you ADD another drive.
You don't need a hard drive caddy for you're ssd you can put one behind the superdrive with the brackets or velcro or double sided tape.
Good luck! Tt is not that hard(if you have experience in pc building

)!