Adding my own PHP forms to WPMU


I have decent PHP skills but still very very raw with WPMU.

I am looking at adding my own pages(forms) to the existing core but here is where I need some pointers to get started:

->I understand that to load the wordpress environment for your page(or PHP file), the wp file ‘wp-blog-header.php’ has to be included; any other files apart from this required?

->To store the input data from my PHP scripts, I would like to add some extra tables to the existing WPMU d/b. So, tomorrow if the user were to upgrade his WPMU installation, he would also require to run an additional MySQL script for those addon tables. Is it a bad idea to keep the original wpmu tables and the extra-tables in the same d/b when they can in fact be maintained separately?

->This is a more specific question than the above ones – I want to save a lot more information related to every blog than what is being currently stored default by WPMU.

For eg: every blog has a blog_id, blog_name, domain etc stored with it.. but I would also like to store the blog owner’s personal/professional details etc which is done by redirecting him to the custom-designed php pages after his blog is activated. For this, I know the ‘blog_id’ is the the primary key to map every blog with its corresponding data, any other fields that I might need to store in addition to the blog_id?

-> Last, but not the least, to what extent would my modifying/extending the core affect future up-gradations and what are the common things I need to be aware of while playing around with the core scripts? I already modified wpmu-functions.php to fulfill a certain requirement, would those changes be lost if I upgrade?

Many Thanks.