The most popular form of floatation is bags, as it's easy to take out and put in after a paddle so they can dry out.

And any area where you're not sitting can be filled if you're patient enough, that's all the way from the ends, to behind the seat, and to just past your feet, some even like to use the floatation bag for a footrest.
As it's just air, and adds no weight it will have no effect on the stability, because it's inside the kayak which should already be full of air.

The only thing the floatation does is keep the water out of the area you've filled with a bag.

Good luck :)