because programs install and media doesn't... programs write information to the registry, like the exact location of their files and if you move them then they dont work because the registry points to the wrong place. Media files don't install and therefore you can move them around all you want but its on you to remember where you put them. (Although windows can and will find them if you want it to but this slows your system down)
So Yes to migrate everything on your HDD to the SD; whats on the HDD has to actually be small enough to fit on the SSD. Pictures, movies, music, temp files, backups, docs, help files and the like can all be left on the HDD. Delete tempory firles, temp internet files, empty the recycle bin. In your email program, if you use one, delete any sent emails you dont need saved, delete the contents of the deleted and junk email folders. lastly uninstall programs you dont use anymore followed by programs you can easily reinstall... until you get to a small enough space to migrate to the ssd.