While not familiar with that specific kit, remember that lambic's are a unique beast. Once you brew one, those Brett "infections" turn in to little buggers. I had mine going in a carboy, and my skeeter pee 6 feet away got infected. So now, my Lambic sits on the other side of the house while it ages. Also, they reside in all the little scratches that reside in a plastic ale pail ... but the pail is relatively cheap in the long run.
Short answer is that you shouldn't expect as much krausen as a high gravity beer like an imperial (stout, I assume). But that depends on the yeast. The easiest way is to set up a blow off tube, just to make sure. But, to be honest, I'm bad at that myself. What I will sometimes do is stick it in my bathtub and put a wine bucket over it. I know, it won't prevent a problem like a blow off tube would, but it is good for preventing disasters.
Also the need for a secondary is suspect at best. You will find lots of people who feel very passionately one way or another. Personally, I would rack it off of the trub, because you intend to have a long aging lambic. Normally, I don't do secondaries at all, and don't think it impacts the flavor at all. If you wanted to serve it in 6 weeks (unfinished for a lambic), than fine, leave it on the lees. But, in this situation, I would rack it just to play it safe. It won't do any harm, and it could potentially do a lot of good.