Is my Startup Disk Compatiable with 21.5” Late 2012 EMC 2544 iMac?

Not sure it offers that much benefit here.

Your new iMac only has one SATA port which likely has a HDD mounted which is likely larger that your 500 GB SSD. While your new iMac can support two drives the second drive needs to be a special blade type SSD as you can see here unlike your older system:

If your system is setup with a Fusion Drive then it already has a SSD mounted.

Keep in mind the OS installed on your older SSD is too old to run your system. So you would need to upgrade it to the newer Sierra or High Sierra MacOS if you want to make it bootable for your new system. I would point out if you got an external Thunderbolt case to mount your SSD to you can gain still more performance! Then boot up under it.

OK, what will it take to put in the 2.5" SSD.

First if you haven't taken off the display of the newer 'Thin Series' iMac's review this guide: Sostituzione Display nell'iMac Intel da 21,5" EMC 2544 It's a bit of work! And if you're not careful you can damage the display! You'll need to get the proper tools which is part of the adhesive kit: [prodotto collegato mancante o disabilitato: IF173-005] and iFixit Opening Picks (Set of 6)

Here's the full IFXIT guide to get to the HD: Sostituzione Disco Rigido iMac Intel 21.5" EMC 2544. The only problem you'll have here is the SMART services in this drive is quite old and I suspect the system won't be able to interface properly so your systems fan will run hard.

Frankly, I would spend the effort going with the custom Apple blade SSD, while a more expensive solution will give your system much more performance. If you really want something internal. Otherwise I would leave the current HDD alone and just get the Thunderbolt case to house the SSD. If you get a multi-bay case you could get a second SSD and then RAID them to get even better performance than replacing the HDD out and you don't need to open the system!