Migrated to MU- now the questions

I've managed to move my 4 blogs into one MU installations and have a few questions:

1) I've noticed that two of y blogs have a /files/year/month directory structure for uploading pictures while the other two have everything placed under /files/upload. Where can I change these settings to make sure all blogs have the year/month directory structure

2) I dont want to make all the themes available to all the users, yet I want some of my blogs to use themes that are not available to general users. As an admin, can I make themes not available, yet assign it to one of the blogs?

3) Where can I modify the "Hello World" post on new blogs. I would like to change this so new users have links to their back end along with a small explanation/tutorial

I'm sure I'll have more to come but would appreciate help with these to start with.