The bureaucrats over at Skywalker Ranch and Lucasfilm have pretty much made "what is canon" into a science. It is considered part of the "official canon" like much of the other licensed work done in the Expanded Universe
. TFU is even more so since Lucas himself gave it the personal nod of approval and worked on the story with the crew at Lucasarts. However to many of us, the films themselves and the info within are all the "canon" they need.
As for my two cents on the whole thing . . . I like the story behind The Force Unleashed and do not see any significant plot holes with the rest of the saga. By the way the story is set at least two years before "A New Hope" (I think maybe even three years). So the Rebel Alliance would be active for some time before the battle at Yavin IV. Not to mention that small rebel groups where forming all over the galaxy at that point, all the Alliance did was uniting most of them under a single banner and cause, creating an organized army that could oppose the Empire on an almost equal footing.