Does DeskScapes only run Dreams animated wallpapers?
Yes but I know you can run MPEG and WMV files also if you have Vista Ultimate and DreamScene installed. I'm not sure if this applies in your case.
If "outside" animated wallpapers can be used, how?
Depending what file format they are, see the above answer.
Also, does Deskscapes have a printed manual of any sort?
No