That's why I recommended getting 2x 2.5 gallon carboys. You want as little headspace as possible in secondary. It's not an issue with primary.
Secondaries are not necessary by any means..I just suggested it as a way to split the batch into 2 different versions..1 with oak and 1 without.
I have also heard that oak chips will sink into the trub/yeast and they stop working at that point..however, I have also heard that oak chips float, so I'm not really sure either way. Were I to use oak chips, I would put them in the secondary and then rack on top of them. Also, they need to be soaked in some kind of alcohol beforehand..vodka will give you a clean oak flavor, where bourbon, rum, or whiskey will impart more flavors into your beer.
Another experiment would be to split the oak chips and soak half in bourbon and half in vodka. The vodka chips should be more traditional to a Burton style IPA. I've had some really amazing bourbon barrel aged DIPAs (I know it sounds odd, but after some time it's fantastic!)