This depends on what you have for resources. If you have a dedicated brooder apart from your main coop/run, then go for day old chicks. It won't cost as much as 8 week olds, and you are less likely to have problems with introducing diseases into your flock. With 8 week old teen-age chickens, you would want to quarentine them first to be safe. Many small flock owners do not have the facilities to quarentine properly. So day olds will be cheaper in the long run, AND safer for your main flock. Stay away from Straight Run, and go for sexed pullets. Or just buy a Sex Link breed - That way, you would be 100% sure of getting pullets.
As far as your possible Roos - at 7 weeks you still cannot be sure. Not until they crow. Too many times you are just SURE that you have a Roo. And then "he" lays an egg!