(note heavily edited after more experimentation)
I've played around a bit in creative mode. Although siggyboy's original design is very compact and elegant, I feel it is somehow difficult to control / understand what goes wrong because lanes are shared between resources and processes, so it's not really clear where backlogs or shortages come from.
What I wanted to do is have a setup that is more easily expandable and controllable, and have a clear "process flow" in the belts, with belts backing up shutting down the non-essential systems rather than causing a spiral.
This is what I came up with on the first attempt:
Reactors are on the bottom, support plants are at the top, all in tileable rows.
All input is from the left.
Most things should be balanced automatically, but there are two circuit controls: if there is too much fuel, some of it is siphoned off for expansion. If there is too little fuel, uranium from the enrichment plant is added to the system.
For a stable system, it should not output any fuel as the uranium is choked off once there is >10 fuel in the center chest.
If you want to breed more fuel, you can set the refined ore inserter to always put on the belt, in which case the setup produces fuel as long as the load is not maxed out.
You can also breed more plutonium, in that case split the U evenly over enrichment and MOXX and permanently enable the refined U inserter, and siphon off the Pu in the same way as the fuel (because otherwise the Pu overdose will stop MOXX and you get a U overdose, which stops reprocessing, giving a spent rod backlog).
There are separate electric circuits for the plant and the turbines, the plant takes about 10MW to run. Of course you can connect them, you can also create a 10MW solar plant to be sure you always have power for the plant itself.
Explanation going counter-clockwise from bottom left:
1. Breeders and fissions reactors take fuel (and depleted U) and produce spent rods (and Pu)
2. spent rods are reprocessed into Pu and U
3. The enrichment plants add U as needed
4. The MOXX plant converts Pu + U (and iron) into fuel
5. If there is enough U it goes on to the enrichment plant
6. Enriched fuel and MOXX fuel combined and split over the reactors. MOXX fuel is pass through a chest to measure and remove as required.
In general, I like this setup myself because it makes the working of the system very visual, and you can easily see what backs up and what is choked. You can easily expand the system by just adding breeders / fission plants / MOXX etc plants on their lines. I've not tested what happens if you break the ideal ratios, I don't think anything bad will happen as long as you have enough reprocessing and MOXX capacity.
[Edit: for my reference: 24 + 24 reactors -> 6 reprocessing + 2 chemplants + 16 MOXX + 4 enrich + 1 fuel]
(but not very realistic...)