That being said, seeing how many times ranma-chan was tagged with her full name without the implication, I believe it's fair to assume most people do expect her to come up when looking up saotome_ranma. They are unarguably the same character after all.
I don't think that tracks as well as it might seem. When looking through tag histories, you'll see the same few names coming up again and again where ranma-chan was also tagged with saotome_ranma, and more than a few of those are the same handful of users adding saotome_ranma to ranma-chan posts that were missing it, which would also mean that some users either didn't think they should be tagged together, or assumed an implication already existed. As it stands, only like 1% of the userbase actually does 99% of the site's tagging, so they're not really any kind of representative of the actual majority of users.
Further, most of them are just copying what they see other users doing without thinking about it. Taggers often don't think about search results when they tag, they just mindlessly copy tags from other posts without ever asking anybody or reading wikis. The Related Tags tab has been a great tool for taggers to horribly mangle every tag we have.
Really though, just cause some users think they should be tagged together doesn't mean it's actually the best choice to do so.