Sour Advice

I have just bottled 1 1/2 gal from my first Flanders Red and am very happy (and surprised) with the result. Now I have 3 1/2 gal left in the carboy that I want to continue to sour. Do I need to worry about the added headspace, should I rack to an even smaller carboy, if so will it continue to sour? And if I did rack it to a smaller vessel I was thinking of adding 3 lbs of cherries. Any advice or insight would be greatly appreciated.

Thanks,
James