you can add your existing pages into sitefinity as external pages. they will be linked to and included in the sitefinity system, but you cannot edit them in the sitefinity editor. If you want to be able to edit pages in the sitefinity editor, I am fairly certain that you will have to refactor your code into user controls.
intra site modules are developed as user controls as well, the only difference is that the administration side of the module (such as adding items, changing descriptions) is done within the sitefinity administration section (as opposed to a custom admin section with its own login). pluggable modules would probably be the best approach if you want to use source control, although if you dont need the sitefinity admin integration, I would suggest you look at external pages as a possible option.
our city site does this. most of our website is sitefinity pages, with all content either being dropped onto the page (such as blogsview, generic content, etc) or custom user controls dropped onto the page in a similar fashion .however we have a separate section that is part of the same domain, but all the logic is self-contained, so we just linked to each page as an external page, even though the pages are themselves physically located as a subfolder to the main site. I hope this makes sense, if not let me know and i'll give you more details.
regarding your last question, yes if you make a control generic enough, there is nothing stopping you from reusing it anywhere on your site.
hope that this was helpful in some way!