These page styles can be used inside a Page Block without issue.
- Contact Form
- File Sharing
- iFrame — make sure that the block size and the iFrame size are coordinated.
- Movie Album
- Photo Album — You cannot have more than one flash slideshow.
- Styled Text
- Loghound Plugins
These page styles can should NOT be used inside a Page Block.
- Blocks — That would cause recursion, deadlock and all sorts of other bad infinities. Try using Master Pages instead.
- HTML — Redundant (and because they are internally different than other pages).
- Offsite page — It just doesn't make any sense.
- Stacks — Never import Stacks or Blocks into any other page style. Nor even into another page of the same style. These layout page styles are designed to layout an entire page. Breaking that assumption is very likely to lead to broken pages.