Information is likely to be added in various places to the Sakaipedia. Content contributors will use their best judgement for placement according to the taxonomy established by page hierarchies.
This is where the Sakaipedia moderators come into play. I hope to recruit moderators for the major sections of the Sakaipedia to help organize it better, to help develop display and organizational conventions, etc. It's very easy to move an article from one place to another in Confluence. If it makes sense for it be in another place, then move it. Otherwise, just cross reference the page from where you are.
Mail is a good example of the organization is non-obvious. Part of mail have to do with configuring Sakai and should belong in the Admin section. Some of the details may get sufficiently technical to warrant being in the Developer section. Each should appropriately cross reference the other material. What we want to avoid is duplication of information. If you see duplicates, merge them and refer from both places. Note however, that I have started a "definition of terms" collection that is intended to be simple explanations of acronyms and technical terms. These are not intended to be the definitive article on the topic, but rather allow for lightweight definitions that might link out to more detail. Terms should not have children, in general.