First and foremost, Shanks and Buggy were on the same crew as kids. The captain of that ship is shown in the anime and not the manga. I do not think that their captain was meant to be Whitebeard one way or another though.
Secondly, Ace is the commander of Whitebeard's second fleet (Whitebeard, I assume, handles the first fleet). Blackbeard was a member of that fleet and, while there, killed some other crewmembers. Ace has taken it upon himself to deal with Blackbeard and is now searching for him. I am not sure if he could take out the posse that Blackbeard has with him. Ace may know they exist, but whether he can take them all on at once remains to be seen.
The information on Ace's and Blackbeard's connection is noted here (http://groups.msn.com/onepiecemanga/onepiecechapter159.msnw?action=ShowPhoto&PhotoID=3131)).
I personally think that Oda meant Blackbeard to be a real pirate as a number of other people have mentioned. I believe that is why he gave Blackbeard, a "villain", an actual pirate's name and not Luffy. Blackbeard is merely a "real" pirate following his dreams. He could have the same dreams as Luffy (gather a crew of ten people and get One Piece, which would definitely see him confront Luffy and Whitebeard's crew later) or a polar opposite one (destroy One Piece to ensure a never-ending quest for dreams and piracy). Blackbeard, Whitebeard, Ace, Shanks, and Buggy are defintely the pirates I want to see after the current arc is finished.