Tall hexagon tanks are possibly the worst possible aquarium ever designed by man. Only a goldfish bowl is a worse place to keep fish! I'd think extremely seriously before spending any money on this system: it will probably be good money after bad.
Gobies rate surface area since they don't swim about in midwater much. For each BBG, you will need a suitable cave (for example, an empty shell) surrounding by a territory about 10 cm in radius around that cave. BBGs will chase rival gobies within that area. So simply look at the surface area of your aquarium and see how many roughly 20 cm diameter circles you can fit on the bottom; that's your answer.
As for "clean-up crew", do always remember that only marine aquarists need these, because only marine aquarists are unable to siphon up the sediment and around the rocks for fear of damaging corals. In brackish and freshwater tanks it is always better to do the cleaning yourself. No algae-eater will actually stop your tank getting algae problems if it is predisposed towards such problems. So for example if your tank has poor water circulation, weak lighting, direct sunlight, and no/few fast-growing plants, then blue-green algae will grow, no matter what you do. On the other hand, if you have strong lighting, lots of water movement, and plenty of fast-growing plants, little algae will grow beyond green algae, and that is easily managed with a few nerite snails and shrimps.
When it comes to detritus, understand that adding any fish or shrimp will always create more solid waste, not less. Adding fish and shrimps will also increase the amount of ammonia in the water. So the net result of adding "cleaners" is always a dirtier tank. If you have problems keeping your aquaria clean, review issues such as mechanical filtration, water changes, and how regularly you stir and siphon the substrate.
Cheers, Neale