your right the rootd do grow sideways, but they are very easy to split of from the main plant, I did this with my black bamboo last year and just stuck them in a spent compost bag with some soil. They have survived the winter and are looking really well. A great way to get more plants and if you have no where to plant them then consider selling.
I would would plant the way you are considering. If they do start to get a bit cramped you can lift the pot and cut through the root and hay presto even more free plants.
Hope this helps