First there isnt anything said that you need rank 6, only you need the right rank to get a house!

Than there will be place for 30 houses in one instance. All three size combined in one instance. If the limit is reached a new instance will be open for another 30 houses and so on. In one instance there will be space for a fixed amount of small, medium and large houses so one instance cant be full of large houses. 10 small, 10 medium and 10 large house = 30. They will look if its nessecary to open another instance if only large houses for example would be bought. So small and medium wont be full. Than they make new instances/space.

So the new instance wont be open if there are for example 3 small, 5 medium and 10 large houses already bought. SE will than decide if its ok to open a new instance or wait.