The problem I have with the Flood can follow idea is that why didn't they after the Halo's were used? Earth was saved and the Forerunners knew where Earth was. Why didn't the Flood go to Earth?
The only reason I can think of for that is that the Flood have a short lifespan if they aren't feeding. Since they have the short lifespan when not feeding, they can't travel as far as Earth (even though they could already travel fairly fast where Earth is almost a skip away). If that's the case though, why didn't the Forerunners just pull a Battlestar Galactica and keep moving for a few months? It's not like they didn't have the time to build and plan ships that could transport their civilization since they had the time to research and build seven Halo's.
