Hi there:
I would think if she had the flu, the poop would be really runny and possible throwing up.
Normally a reaction will show up within the first 4 days.
That is a lot of times to be going to the bathroom though, espcially since it is waking her at night.
How is her mood? Any rashes or anything else weird going on?
I think you would be okay to try the mixed cereal. They do say to hold off on wheat until 1 yrs old, but if you read the ingredients of a lot of the teething biscuits, snacks, etc. they all have wheat in them.
Since her poop is not really runny, I don't think you have to worry about dehydration. It may just be her body getting used to a new food.
If it still continues though for a week or more, I think I would stop the new cereals and go back to just rice for a while.
Hope it goes well, Wendy