Is the SATA Connector from older iMacs Compatible?

Both 2.5" & 3.5" HDD's and SSD's all share the same SATA data cable interface. The 2.5" cables use a unitized design whereas the 3.5" drives often had a separate power line.

Apple did contract the HDD makers to a special version of 3.5" HDD from 2011 to 2016 as Apple uses the in drive thermal sensor within the systems SMC services to manage the systems cooling. Apple altered things once more forgoing the custom drives leveraging the updated S.M.A.R.T. services which they championed but took time to get get done. To leverage the S.M.A.R.T. services you need to use hither newer Catania macOS and its firmware or newer, otherwise you need the OWC thermal external sensor. At which point any HDD or SSD will work.