hitbykamikaze --
Single board in med onc will to some extent limit your opportunities post fellowship, but you will still find a job. The issue is, hospital systems are increasingly requiring various credentials for employment, privileges, etc.
Gutonc is 100 % correct in that even if you are not boarded in hem, you will be expected to do it. In most parts of the country, PCPs, Ob Gyns, etc and others know "hem onc" as one thing --- your predecessors did hem and onc, gleefully gobbled up anemia consults and if you don't, you will be perceived as a bad egg.
Transplant fellowship will not help you in community practice. Transplant fellows go into community practice all the time, never to transplant again.