Running a function before a View executes
You might be interested in a similiar kind of question I answered here:

How do you customize the title of an RSS feed created by Views RSS?

I would just set the view size to 10 always. And then use array_unshift() as I did in that reply to "prepend" the new item Object to the front of the view results array, if your Twitter call return its 1 item of content. Or else you just let the original 10 come from Views.