You could have the entire thing made of acrylic. Then you wont need to buy shelving. Everything could be self contained into one giant unit. For a community tank such as a barracks you will need to run UV so your fish don't get each other sick. Also I do not like wet dry sump tanks. They take up too much space and are not really efficient. You will have ammonia spikes during feeding and have to deal with channeling and flushing out the media every now and then to keep things working.
As for the video that was posted. I think that having all your tanks connected and having water flow threw from one tank to the other across the bottom is the best solution. I have done this and the reason I did so was that it is easier to maintain the tanks this way. I just increase water flow and all the poop gets flushed to the other side where you have a collection area then you just reduce water flow again and suck out the poop from the collection chamber. I did not like her methods for creating water flow, its not necessary. The back channel may seem like a good idea but it takes up too much space. It's better to have the water flow across and flush out the waste from their tanks. In her video I saw poop just sitting on the bottom of the tanks. The gap between the dividing walls and floor only needs to be about 1mm so the betta will not get sucked into it. Water flow should be a slow trickle and create almost no current. You will not have a high turn over rate which is better for the bettas as they will not have to fight current.
As for media I prefer bio wheels. You can create custom holders for the bio wheels so you do not have to buy the expensive housings. The replacement wheels are pretty cheap. If you go with a custom acrylic setup you can have them create a sump area that will hold multiple bio wheels. Water flowing down a ramp with successive bio wheels in line feeding the bio wheels so they turn and then the water can collect in a sump Or you can just have a wall with bio wheels in a ladder type setup with a spray bar feeding the top wheel and the water that flows down should hit the next. Might have to stagger them back and forth so that the wheels will turn in opposite directions feeding the next bio wheel. With this type of setup you will not need a giant sump tank.
Just tossing out some ideas. Anyways good luck I hope to see what you come up with.