All folder IDs are assigned by the server, and so you cannot choose the ID for your folder.
Any type of folder can occur anywhere in the folder structure. So yes, you could put a calendar under Inbox. (I'm not sure if the web client allows this, but the server doesn't care.)
Folder names must be between 1 and 128 characters long. They may not contain control characters, tabs, carriage returns, linefeeds, or any of the three characters " and : and / |