OK thanks just saw a video of someone who had 2 drives and didnt use the ssd and it got me thinkingYou can use the boot drive for anything you want, assuming you have the space on it.
Your SSD should have windows and all your programs installed, what you do with the rest of the space is up to you. Just be sure to not pass 80%.
SSDs are great to keep games on for load times, or extremely commonly used files that require fast load times.
Generally though, files/pictures are best on an HDD simply because SSD speeds arent needed.