I read that in the Constitution it was stated that if Canada wanted to become part of the US, they would be accepted fully. First off, is that true? Building off that, if they wanted to be part of the US, would they be accepted, or would they be kinda, but not really (think of Guam )?
No, that option expired in 1788.
The Articles of Confederation had, as clause XI:
Canada acceding to this confederation, and adjoining in the measures of the United States, shall be admitted into, and entitled to all the advantages of this Union; but no other colony shall be admitted into the same, unless such admission be agreed to by nine States.
When the Articles were replaced by the current Constitution, that clause was dropped, since Canada clearly wasn't interested.