How and how much do the dolls know about the other dolls?
The question "How and how much do the dolls know about the other dolls?" is hard to answer by the anime perspective.

The anime storyline started to differ from the original manga storyline, and the differences became great enough that they can be considered different works.

Not many details are included about the dolls' past in the anime, and the plot offered in both Träumend and Ouvertüre was discontinued, as they were animation original plots, not related to the manga storyline, so what we have seen is all we have to try and deduce answers for that specific scenario.

The second manga series advanced the manga story, and gave more information about the Rozen Maiden's past. And it contradicts and negates the information given by Ouvertüre.

It is revealed that, at first, the dolls all lived together in what they called "The Miniature Garden", altought it was more of a big deposit than a real garden. They spent time together, playing around.

Yet, events happened, and while they expected their creator to be making a seventh doll, neither the seventh nor their creator ever appeared, and as they started to question wether they had ben abandoned and forgotten, LaPlace appeared before them and started the Alice Game.

So, by the manga plot, the six know maidens all knew about each other, because they lived together as sisters before the game began. But they never knew if a seventh really existed.

But the manga plot also does not have a Bara-Suishou. She exists only for the anime plot.

As for the actual seventh doll, the reason none of the maidens met her over the years, is because the seventh doll did not have a real body, so she was not able to properly participate in the game.

Sorry, but since I still haven't finished reading the manga, this is all I know at the moment.