So after you join, go meet other teams and get to know the big picture. Then, you can seek a transfer to wherever you think you'd best fit in.
This is the best way to align everyone's expectations.
If you approach situations like this in a constructive and professional manner, the company will go to great lengths to find a solution. At least that's been my experience.
i had potential teams lined up (one of which I would've been allocated to initially, had they not hit their new grad hire cap), so it was certainly a very frustrating experience. perhaps they care more about established engineers than recent graduates?
3 weeks of conversations later, I was told that the 18 month rule was "fair" in everyone's opinion, and that if I'm really concerned about working on projects that are interesting to me, I should just find one to spend my 20% time on (easier said than done).
This is always good advice whether the offer is from Google or a Big Bank.
Talk to the manager, talk to your coworkers. Once they give you the offer, their job is to market the job to you. Use that as an opportunity to interview your future coworkers. When I was considering finance job, I've even gone as far as asking the future coworkers to implement different algorithms and/or talk about their system architecture. If it was too crazy, it was an immediate sign that I won't be happy there.