I have a question on the rule about no posting unrelated content on posts. Is writing things such as "This is my 500th post" allowed? Would that be considered unnecessary as it has nothing to do with the topic of the thread or blog post that you are replying to or creating? I think that this might take a mod or an admin to answer this question.
if it's a new post about your 500th post, what's it not related to? it's not like you hijacking someone else thread.
frihost is not really draconian about what you post anyway. everybody will love you when you open a new thread about your 500th post.
I think if you answer and make good post related to some topic then no one here will not think you are spamming if you write "This is my 500 post" on the end. But if you write only this sentence then is better to start new topic where people can congratulate and post some answer related to your 500 posts.