Can I work in CS5.5 & save premiere project to open in CS5.0.3?
There is no explicit backwards compatibility. It may work, but probably won't. I would not count on it to work. You might be able to work around with AAF or FCP XML export, however. Again, not an ideal workflow.
You can probably safely go from CS5 to CS5.5, but CS5.5 to CS5 is taking a risk.
Photoshop and Acrobat deal with documents. Adobe's production software handle the intricate world of video. AFAIK, no files saved with a newer version of PP or AE has ever been compatible with earlier versions.
AE CS5.5 can save an AE CS5 project.