If you keep the 2 BD's and 1 skimmer, I'd add a couple of 4" egress points (like mid water pickups) and plug them for
future use just in case. If you ever decide that you need more flow to the filtration, without more egress points, you'll
be pretty much stuck. The 4" BD's should flow about 3500-4000 gph each and the skimmer with an 8" weir and plumbed
with 3" or 4" pipe should flow another 2000 gph or so.
Most RDF's that can handle that amount of water are around 3 ft. in length so by the time you plumbed the pipes and valves
in the concrete chamber I think you're going to need more space than the 100 cm the first two will give you.
The best bio filter I've used is a shower filter, but they do present their own challenges... so if you want to keep the
gravity fed moving bed bio filters I'd just make a single large chamber after the RDF and use the 2 - 4" outputs
from the RDF to feed it. I don't think multiple bio chambers will help any and will only cause more problems with draw down.
If using media like K1 for the bio filters you'll need some way to separate it from the pump outlets to flow +8000 gph.
The RDF removes any need for the brushes or other mechanical filtration.
Just curious, what RDF is pictured?