can i delete downloaded stuff from command center?

what you have in there are all the updates you've downloaded, no surprise there.
if you have rolled back and like a particular version, keep those.
if not you can pretty much delete all of it because they are only the installers.
upon restart sonar will recalculate the cache.
 
if I were you though, keep a one, or two of the latest updates incase you need to do a re install if something goes array (you'll have something to fall back on).
 
you can also do a search, this has been asked many times (even as recent as last week)

thank you very much guys!