Sadly, you're between a rock and a hard place here. As the model you bought came with a HD Vs a SSD to start with. You may want to think about getting a replacement system which has the SSD and then transplanting your HD in to it Vs trying to open the system to add in the SSD. Then get another HD to put in your system and sell it to recoup some of what you spent.
While its possible to add the SSD, its a lot of work and and you have discovered not easy to identify the correct SSD to buy to put in. I should point out the SSD's also have firmware on them which also limits which system it will work in. So getting the correct looking SSD (even by vendor P/N) may still not work! We have the newer Mac Pro systems and we also tried to locate alternative SSD's for them, of the four we bought we only got one to work.
Review these two IFIXIT guides:
* [guide|28781|iMac Intel 21.5" EMC 2638 Hard Drive Replacement]
* [guide|29130|iMac Intel 21.5" EMC 2638 SSD Replacement]
Note the number of steps needed to replace the HD 29 and 58 for the SSD. Are you up to this much work? Do you have the needed skills and tools?
OK, as to using an external SSD to marry with your internal HD: I would hold off that direction. Instead I would get an external dual drive unit and then use one of the bays in it to hold the SSD and marrying it with the mate HD so the external drive is setup as a fusion drive. While this will be more expensive it will offer more stability and speed than spitting across the internal HD and external SSD.