Being that it is a 6 gallon carboy, you could probably get away with just using an airlock, but if you have a 3 piece airlock, all you need is a hose to attach to the center post inside it to make a blow off. Better to have it and not need it...
I've never used the carboy caps, but I would assume that would work too. As long as you have an airtight seal. Maybe someone else can comment on that.
*Edit: Wow I'm slow. oh well
Secondary #1: Mead Experiment
Secondary #2: Welch's Wine Experiment
Kegged: Organic APA, Kick the Bucket APA, Pub Ale, Argentine Cascade IPA, ESB
Bottled: Wild Raspberry Cider, Cyser, Insane IPA, Lady Bug Pumpkin Ale, Cider, Christmas Cider.
RIP: Raspberry Hefeweizen, Hoppy Lager, California Common, Oktoberfest, Hefeweizen, Melted tubing brown ale, Honey blonde ale