No dude, you dont need to put your steam folder fully on the ssd.
Having os and apps on there is brilliant. Then do what me and zarf did. Create the steam file on a partition (the first one so it is shortstroked there) on your platter drive.
Games that you play often, say your top 5, you copy and paste the data files out of steam on the platter and onto the partition on your, say 80gb ssd.
Then using command prompt you make a symlink, a kind of shortcut for programmes, that will sit on your platter drive where the main games directory is, when you go to access the game, the OS will be told to look on your ssd- this gives ssd performance, for all the apps and games you use.
On an intel I have OS, apps, civ 4, napoleon total war and battlefield 2. Still have 23 gig spare. I spend 95%+ of my time using only the data on my ssd.
My platter drive holds the backup of my steam folder and the rest of it plus my data. If you do want to use a game you dont play so often, you can just play it off the platter- odds are it ll be an older one and it will load very fast anyhow.
All the benefits of an ssd, but you use far far less capacity, and trust me, you notice the difference.