The hardrive connector is Sata 1.5 Ghz, which menas that you have to find a SSD which is downwards compatible to 1,5GHz. Modern SSD's are 6GHz sata and several have difficulty to downclock to 1,5 GHz.
The internal CD slot is even slower, so if you want to put a SSD in you should put it in the place of the HDD, and not in the CD connector. You can put the HDD into the CD connector.
Try to find a Sata2 SSD in the OWC shop.
I have built-in 4 SSD's in four years).
I would recommend OWC 3G SSD drives. I recently installed one of their 6G drives in a 2010 mini, which only ran at SATA I speed instead of SATA II. The solution was to use their 3G drive, which works fine.
You don't need to pay for an SATA III capable SSD, and probably can't find an SATA I capable drive now. The OWC drive is an SATA II drive, which you could confirm with OWC that it will run at SATA I speeds without issue. OWC does give you 30 days to use it and return it if unsatisfactory. 3 year warranty too. They also offer a bundle with an 5.25" drive bay adapter, or you can buy separately, as it's a common part.