Well, yes, you would need plug the hole with the airlock, or a bung or something. The kraut needs to be both under the liquid entirely, and in a relatively airtight container. This just makes it easier/cleaner. Realize that the old crocks weren't airtight at all, they just have heavy round boards to keep the kraut under the surface of the liquid when it floats up a bit. It doesn't really float that much after awhile, so you can push on it if/when you see any bobbling up.
A couple of issues:
1. A six gallon bucket will hold a lot of cabbage, a lot more than you will eat in months, unless you give it away. And it's a lot of kraut to waste if your first experiment goes bad. But cabbage is cheap, so if you feel like doing a big batch, go for it.
2. The cabbage needs to be kept away from air, and it floats somewhat at first. So you want as little airspace above the kraut and the top of the container as possible, or you can also put a plate with a weight on it to hold the kraut under the liquid. So hopefully your bucket has a lid that comes totally off, so you can either fill it to the max, or get a weighted cover in there.
3. Plastic does absorb some odors/flavors after time, especially the length of time and the power that kraut has. If you want to use this bucket again for beer, keep that in mind - it might not ever be useful for anything but kraut ever again!
I prefer to buy containers just for kraut making and nothing else, but you can play it anyway you like!