... the only one you need is Quicktime 10.
While I don't use SL myself (never jump the boat too early..), I did some reading about it. From what I've seen so far, I got the impression that for many tasks QT7 will be needed for quite a while. Simply put, QuickTime X just isn't finished, yet.
See "Mac OS X 10.6 Snow Leopard: the Ars Technica review"
by John Siracusa
The part about QuickTime X is here
It is not that the features are not there, it is because in the rewrite of Quicktime the interface for plug-ins was changed. The third party plug-ins have to be rewritten to conform to the revised interface..
Looks like the new interface doesn't exist, yet. Here's a quote from the article linked above:
It also supports only "modern" video formats—basically, anything that can be played by an iPod, iPhone, or Apple TV. As for other video codecs, well, you can forget about handling them with plug-ins because QuickTime X doesn't support those either.