related posts based on one tag- ordered by categories

Hello,
first thanks for havin WordPress.

While developing a new site with it Im am looking for the possibility to display related post links based on one filtered tag ordered by categories.

What I mean: there is a topic tagged A,B,C,D, where A is the main tag (=topic identifier) and i want to display links to other A=topic related posts ordered by category (ie. A in News, Reviews etc.)

While I am using the Simple Tags plugin, it doesnt filter me the first tag, Simple Tag displays all tags (or all related posts), ordered randomly.

Already found a work around splitting the A tag archive into its categories (http://wordpress.org/support/topic/152904), and thought I could use it also on a single entry page, but cannot call the $current_tag = single_tag_title(“”, false) there – no result. Obviously it doesnt know which tag to pull? Thats why I tring to find a way to filter only the A tag.

Any idea?
Ive been searchin this forum for hours…
Thanks a loot!

ps. tried to figure out the following pages, but didnt give me clue:
http://codex.wordpress.org/Function_Reference/get_post_custom_values (for using an extra custom field for A)
http://codex.wordpress.org/Template_Tags/single_tag_title
(doesnt give a result)
http://wordpress.org/support/topic/157709
( Only use first tag?)