If you don't do RAID, it must be a factor. You will have a fast and a "slow" disk
The SA500 is compatible (https://www.synology.com/en-eu...)
The 4000GB HDD see the model.
As Darxmurf said, all drives should work, but it's better to put a model from the compatibility list ;-)

Look into making a JBOD configuration. Mixing SSD and HDD in a RAID is not a good idea.

We believe that the SA500 is not compatible. A full list of compatible HDD/SSDs can be found on the manufacturer's website: https://www.synology.com/fr-fr...