I think your idea to partition the SSD into a smaller root and larger home and use the HDD for large data files (movies, backups) is perfect. At least that is exactly my setup.
/root has to be on the SSD (because this is where the SSD will be most advantageous), and clearly, the root partition does not need much space. Actually, 20 Gb is still an overkill -- I have 15Gb and still over 3Gb free with tons of software installed.
Also, if you plan for a swap partition, put in on the SSD.
/home makes perfect sense on the SSD, as there are tons of user-specific configuration files that need to be accessed quickly.
Finally, if you run any kind of databases you might consider creating a database partition on the SSD. SSD are not much faster than HDD when it comes to data transfer rates, but they beat HDD by order of magnitude in terms of access time -- and this is often the limiting factor in databases.
As for the mount point: you can make your HDD mount for example to
/data, and then create symbolic links in your
/home that point to a particular directories on the
ln -s /data/Videos ~/Videos/