That requires neanderthals to survive too.World War II would be determined by two of them sneaking into Germany to drop Hitler's ultimate weapon back into the volcano in which it was created. Then they'd return home to find that Mussolini had taken over their country and they'd have to fight a war of liberation.
I'm really not sure can crossbreeding even happen between a homo floriensis and a homo sapiens.
http://en.wikipedia.org/wiki/Orang_Pendek
This could be reality. If you believe in cryptids.
Apparently the stories of the locals have these creatures kidnapping children, the only humans they could reliably overpower. They would hold the children and try to get them to show them how to make fire, or other tricks we can do. The children would always be able to outsmart the Orang Pendek to get away, though. Creepy, with shades of changelings and 'King Louis'.
So if the stories are true, it is likely that most of them were already wiped out by the locals before westerners even got there.
I think regardless of whether it is possible to crossbreed between homo sapiens and homo floresiensis, it will be tried.
Repeatedly.
And put on DVDs. I'll leave the titles of the DVDs as an exercise for the reader.