In my experience the 18 months 'rule' is much more a guideline than a hard rule. I've personally seen a number of cases where someone transferred much sooner than that for a variety of reasons.
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.
unfortunately, it certainly doesn't seem like the company does this all the time. i got placed onto a team which has no bearing on my background experience whatsoever, and after approaching my tech lead and some HR people to continue the conversation, everyone else decided to agree that the 18 months guideline was "fair" and that the 20% projects were there to satisfy my need for interesting problems.
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?
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.