Will it do any harm to cannibalise an old hard drive as an SSD enclosure?
The SSD draws a lot less juice from your power supply and doesn't produce as much heat as a spinning hard drive.  You could say your enclosure is quite over-engineered for your SSD.

I would be concerned if you'd try the opposite (putting a spinning harddrive into a SSD enclosure).

That being said it would be better if you'd put the SSD directly into your iMac.  Or swap the drives.  It's not that difficult and you'll probably gain quite some speed.