Oh, I missed one of your questions, you were asking what the Gedoushu are? Well, they are just a fictional race of demons that resides in the Sanzu River which were just created for the show.
Given my explanation on what the Sanzu River represents in Japanese Buddhism, the writers obviously wrote the Gedoushu based on the Youkai (Demonic Spirits in Japanese Folklore) to be the creatures that live on the river even though I don't think the demons living in the Sanzu River according to Japanese Buddhism were called anything as a group or race at all.
The Youkai are just demonic spirits in Japanese folklore which has a lot of categories (Kitsune, Kappa, Oni, etc.), but in the folklore, they don't have any particular place where they reside, not just in the Sanzu River as presented in Shinkengers.
The Gedoushu were written to be the demons living in the Sanzu River which were created because of the sins collected in the river itself (since sinners are said to drown in the deepest part of the river where the serpents live in Japanese Buddhism), hence the Gedoushu always wanting the humans in the human world to suffer and live in negative emotions to raise the level of water in the Sanzu River, giving them more life force and to allow the river to raise enough that it spills from the crevices in the human world so that the Gedoushu can easily live in there without fear of running out of water from the river.
To my analogy, the Sanzu River in Shinkengers isn't exactly the Sanzu River in Japanese Buddhism, for one, in the show there isn't anything resembling the 3 paths. Another point is the existence of a race of demons called the Gedoushu which live off on the sins of man in the river (I don't recall that part about the demons in Japanese Buddhism). And lastly, they didn't emphasize the idea that the Sanzu River is just a passageway to the afterlife and made it seem that the river was already the land of the dead where evil lives and the souls of sinners go to (Dayu, Juuzo, etc.) even though the sinners according to Japanese Buddhism are just devoured in the Sanzu River but the righteous get to cross the river safely and reach the afterlife. So it seems they combined all the ideas of "hell" in past and modern Japanese beliefs to create the "Sanzu River" in Shinkengers.
So let's put it this way, there are 2 types of Sanzu Rivers now:
a. Japanese Buddhism Sanzu River - 3 paths, just a passageway, random demons reside on.
b. Shinkenger Sanzu River - 1 path, seems to be the equivalent of hell, fictional race of demons called the "Gedoushu" resides on.
And regarding the difference with Dayu and Juuzo, it has been talked about in the Goseiger Talk-Up thread:
http://henshinjustice.net/forums/showthread.php?t=48254&page=234
Theories are that Toei wanted to show-off the suit technology with Dayu hence using her costume much more than Juzo's Gedou mode, and mixed it up with the idea that Romi Park had other commitments that's why she can't always be in the set to shoot her human form in the show. So I guess Dayu and Juuzo are just the same, though Dayu stayed with Doukoku since he obviously likes her, and Juuzo is really just a loner. It really that isn't big of a deal, it is just what it is.
Hope it helps.