I don't know about grabbing a "selection" of posts from various categories -- the standard WP "blogroll" lays out the posts chronologically. I wouldn't be surprized if there weren't a plugin that would solve that problem for you.
As far as "little boxes" -- go to "Atahualpa Theme Options" in Appearance, then to the "Post/Page Styling" tab -- I put
border: solid 3px red;
in the "POST Container" box and it created http://www.shepherdjim.com/blogg
-- that red box also surrounds single posts on single post pages. Without the 10 pixels of padding the post text runs right up against the red border.
May 2, 2009 -- BTW -- I'm going to remove the red border lines on my ShepherdJim w/site now that you've had a chance to see them. I did note that the borders were NOT appearing JUST around posts on single and multiple post pages, they were also showing around the text blocks on page pages.
WP automatically creates excerpts (160 words?) for display on multi-post pages, You can stiplulate your own excerpt breaks by inserting the "read more" tag when creating a post. You can format the "read more" in the Atahualpa "Read More" tag tab.
There's more info about "read more" in the WP codex also.