@ thread opener: i think its always wise to have a SSD as system drive for windows and the games folder.
For any data like videos or images that can be archived and do not require quick access it is fine to put them on the HDD.
Honestly I would not think of building a system only containing a SSD, because you will be quickly limited on the amount of space available. Also the SSD performance will go down the more space is used! So it should be utilized maximum at 50 percent. Thats what i can say from my experience.