Hey Goody,
The reason for water being pumped into the individual barracks and then overflow is more so for heating purposes. If I were to live in a tropical climate or have a heated fish room, then I would just have individual barracks sitting on a shelve (that's how I did it in Malaysia) and do full water changes once to twice a week depending on the size of the barracks.
If I were to build it all over, I would make it higher. Looking at the picture, I'd stop at the 3rd level (from the top). I'd have more space for the sump below (a bigger sump could be used) as the bigger in volume it is (to a degree), the heater would work less hard as there is a smaller temperature fluctuation. But having said that, the bigger the volume, the more powerful the heater must be (or use two less powerful heaters instead of one powerful heater - it acts as an insurance in case one heater fails). Also I would use clear containers, either glass jars or plastic beanie boxes, so I would be able to see the fish clearly (I did not know how beautiful some of them are until I decided to take photos). Beanie boxes are better as it is easy to drill holes but care should be taken as it cracks easily. It's limitation is its dimension. If you seen Shemma's setup, he uses plastic drink bottles.
The reason I built mine like this is because I may want to breed more bettas in the future (and have a dedicated fish room), I can use the shelves behind, rig a fresh plumbing system, and have a large display aquarium on the top with lots of plants and shrimps but no fish, which would have water pumped from the sump, into the aquarium and overflow back into the individual barracks. Thus giving the bettas 'healthy' water. Also it would solve some problems with heating as water to the sump is heated to say 26C and water in the top aquarium is heated again to 27 or 28C. But if you have a dedicated fish room that is heated, then there should not be a problem.