is the guid unique across an entire community or unique only across applications?

The nature of Guids is really they are globally unique but the platform sees them as unique within a content type(or application type if we are talking about an application).  Its a better practice to reference the ContentId (I assume this is the Guid you mean by also using its ContentTypeId which is a Guid that says "I am this type of content".  

To spearhead the next question, you can get a content type ID off of the Api consistenty.  Here is an example of the thread content Id and  a forum application type Id.  This also applies to the in-process Api, available in the documents

$core_v2_forum.ContentTypeId
$core_v2_forum.ApplicationTypeId