In short, you should not have to worry about installing QuickTime Player 7. From my understanding, QuickTime Player X can play all the same formats as QuickTime Player 7 could.*
If you do need to install QuickTime Player 7 for whatever reason, though, just insert your Snow Leopard Install DVD, open it in Finder, open the Optional Installs folder, and then open Optional Installs.mpkg from within. Go through the steps, and you’ll be able choose to install QuickTime Player 7.
*If you’re interested, here are the technical details. QuickTime and QuickTime Player are two different things. Both QuickTime 7 and QuickTime X (10) are installed by default in Snow Leopard. QuickTime Player 7, however, is only installed by default if you have a QuickTime Pro license on your computer. QuickTime Player X, on the other hand, is installed by default. QuickTime Player X uses both QuickTime 7 and X, so you should be fine when it comes to playing videos; if they worked for you before, they still should.