I'm currently working on implementing BrokerHistory. IB doesn't allow for getting historical data from an arbitrary period. IB's history is really a backfill so it starts from the current date/time and goes back to a specified end date. Also there are limits on how many bars of history you can request in each time frame.

It looks like FXCM allows requests for any number of bars from any period in any time frame. Is that true?