One loop of rail.
One train per item?
A train pulls off this loop into a "pickup" station for its good, it sits there until a "delivery" station is activated (say because its input has emptied/is low). The train then delivers, and returns to pickup.

So a section making green circuits would have ...

Yes, you sorta have to mess around by using the value of your steam storage (you can read that) and using it (+combinators, deciders etc) to specifically turn on or off the inserters that would feed fuel (and override stack number, so each inserter only puts in one unit).

eg: If you were using 25 ...

Yeah, something like the reactors only go if the steam storage is (nearly) empty, with a large enough steam storage to basically hold the entire output of one fuel cycle. Depending on your reactors though (adjacency bonus) a round of fuel might give quite a lot of steam.

Depending on reactor # as ...