On your question about telling NodeXL what kind of data to import, you cannot say anything like "give me the most central." You can, however, narrow your search using Twitter "search operators," which are documented at https://twitter.com/search-home# under the "operators" link. For example, you can enter this as a search term in NodeXL to get a selection of recent tweets that mentioned "NodeXL" but not "graph":
nodexl -graph
-- Tony
nodexl -graph
-- Tony