Let me try to clarify things for you a bit. Search engines don’t have any rules or expectations about website topics, they generally ignore the concept of a website topic. They are much more granular in their approach to indexing the web. They can detect different topics within an individual page and include that same page in search results for various topics.
While it may be confusing to your visitors to have a narrow focus on a single topic for your website along with pages that are unrelated. This is not really an unusual way to build a website. Just look at sites like wikipedia.org, ehow.com or ezinearticles.com. None of those sites have any problem including many unrelated topics.
Search engines don’t index and rank websites, they index and rank individual pages. They don’t look at your website for relevance signals, instead, they look at the “web” that your page is contained within. This web is made up of your page, along with all of the pages that link to it and the pages that your page links to.
So even though your page is part of your website, it is viewed by the search engines as part of the web that includes linked pages from all across the world wide web. It is much more important to consider which pages you link to and where your backlinks are coming from than to consider the topic of your website.
Naturally, it can be beneficial to link related pages together to attract targeted visitors, however, there is no strict rule that says you must do so.