Tags: customised behaviour

Hi, I’d need to customise how tags work. I’d obviously like to write a plugin to do it.

The custom behaviour I need is:
– Add a section (form) to create tags in the “Manage Tags” page
– Replace the textarea with a select (dropdown box) in the gallery page (image list).

By doing so, I’d allow a role (e.g. Administrator) to create tags and other roles when adding pictures can just select one from the existing tags from the dropdown.

I know that there is no question in the post ;-P
I’d need some guideline, a starting point, to write the plugin and don’t mess with the core

http://wordpress.org/plugins/nextgen-gallery/