Wrong, you want the pressure to be equalized or for the intake the be passive. In a cabinet enviroment if you have the intake pushing less air than the exhaust and you room is sealed then the intake becomes nothing more than an air flow restriction. It would be better to do without an exhaust in this situation.

Also you shouldn't really have any cracks or anywhere odor can leak out so easily. Duct tape is cheap to buy and easy to use on the inside of a grow cabinet.
Yeah, what I meant was that the intake fan should be SLIGHTLY smaller compared to the exhaust fan, but of course they can be equal. It's just hard to get them EXACTLY equal since the exhaust fan is loaded with ducting and the filter. If you try to vary the fan speed, the fan speed controller is an expensive item (sometimes more costly than the fan itself). I suppose you could duct some fan air away to vary the CFM of the intake fan.

I just wanted to emphasize that if your intake fan is LARGER than the exhaust fan (after loading) then you'll have positive pressure in the chamber and may have the CHANCE of leaking unfiltered air if your chamber is not absolutely air-tight.

As for cracks and air leaks in the chamber, they could develop over time and may not be revealed by light leaking out.