Brand can play in a running game. The first step to a fast break is always a rebound and blocked shot/turnover. You don't need 5 guys sprinting down the court for an effective fast break, and having Iguodala, Thad and Dalembert to fill the lanes is plenty.
What's really killing our fast break right now is:
1) We're not forcing turnovers
2) We're not blocking shots.
Dalembert's blocks are way down. Brand's are almost non-existent. Young's getting 1.2 steals per 48 minutes, he was getting 2.3 per 48 last year.
We're just not forcing turnovers. And I do think that will come.
If I'm worried about fits, I'm mainly worried about Miller (a HORRIBLE pick and roll pg, and pick and roll's are brand's best play), and Dalembert (who's really clogging up the paint offensively).