Headcanon: The senshi also didn't learn of their castles until later in the series. Those castles still stood by their home planets even in the present, so I believe they are still there in Crystal Tokyo. The senshi are now rulers of their own kingdoms.

With long lifespans, it's inevitable that the world would quickly become over populated. 

The people of the earth destroyed the people of the Silver Millennium, and their ancestors would repopulate it.

Spoiler:

Now to the reason I stated all that topic unrelated stuff, because this head canon is why I think the outers weren't their.

Obviously they remember that in the past, the inners had gone to the future.

The Outer's were sent away. Wanting desperately to help but like in the original Silver Millennium, they had other duties.

They probably were protecting the other planets from any attacks by Nemesis. Nemesis probably focused the brunt of it's attacks on Crystal Tokyo and probably only on that side of the planet Earth. It was after all because Death Phantom's soul had tainted the planet and they became one. It was a personal vendetta against the woman that had banished him. But just in case, the other kingdoms needed to be protected.

I think all the senshi rule the planets together, like a council. Their bond is like a deep loving family, and they consider and respect and love each other. They would help in each others decisions.

The Council of the Crystal Era, but each rulers of their own Kingdoms, because it's a unity of kingdoms, and one does not own the other but it's like a peace treaty between countries. Allies. Crystal Tokyo may be the capital, but some of the other cities may be more popular like California and Sacremento and Los Angelos or Hollywood lol.

I imagine that other planets in other systems have their own similar Galactic Alliances.

Because like http://www.kinmoku.net/pops/melody/mokusei.php but a little different.

Kinmoku is a giant red planet, but much like earth except minor differences. Lush green lands, and various geological formations but many volcanoes that create jungle like area's all over. The sand and dirt is red, and the oceans, their water, is red.

The Starlights are depicted as heavenly bodies, wandering stars. Like the Asteroid senshi, yet not, they hail from comets, and their homes(the comets) were trapped by the pull of the planet Kinmoku, and as a result, the three lights were in turn pulled to Kakyuu-hime, and like the Sailor Senshi towards Sailor Moon, they devoted their lives to her.

So their comets became moons.

The Alliance in their corner of the Galaxy is the capital Tankei Kingdom(Kakyuu-hime's kingdom) and the various kingdoms on Kinmoku, and the three moons. Fighter, Healer and Maker. (Hey what may be strange outside of Vegas is fun, if only in Vegas. What's weird names to us might be important to them lol?

And who is to say there were mini alliances within the Silver Millennium, that doesn't carry over into the Crystal Millennium? Coronis obviously greatly appreciated Mars, while Mau was specially fond of the Moon. Whose to say planet Mermaid wasn't fond of Mercury and/or Neptune.

The senshi of the Milky Way galaxy are famous all on their own.

Spoiler:

Like planets, asteroids, comets, sun's, floating in space, we also know their are other things. Black holes, warps in space, dark matter. 

Whose to say there aren't other things. Crystals floating around, souls without a heavenly body or a humanoid body? But CRYSTALS.

This is stemmed from another's theory about how Queen Serenity didn't actually come from the moon but a different galaxy like Nehelania. With her she carried the Silver Crystal and brought it to the moon, which is why she doesn't have a natural crescent moon upon her forehead but Usagi does.

I imagine a war raged where Nehelania and Queen Serenity are from, and that they knew each other.

I believe that the silver crystal was a person, and died in a battle or something where they are originally from. I believe, that person was Queen Serenity's favorite person. Best friend, mother, sister, lover, whomever. To save that person, she binded her own soul to this person, to prevent their starseed from moving on to the galaxy cordon, and to do this she pleaded with Guardian Cosmos. With their souls binded, Queen Serenity felt the pull of a planet's soul pushing to bind with her and this starseed(Silver crystal)'s soul.

(If you've ever seen Avatar: The Last Airbender, imagine how Yue was able to live because the moon spirit decided to share it's life force with her, and when it took it back, she died.)

The planet wanted to, originally, bind it’s soul with Queen Serenity because of her selfless act but instead because she bound her soul to the Silver Crystal, she blocked her starseed and it was forced to bind to the Silver Crystal where it still felt the pull of Queen Serenity’s soul because of their connection.

Which is why, a Sailor Moon was never supposed to exist, because although the bind is irreversible, the planet refused to turn Usagi into a senshi.

The only way the silver crystal starseed could gain a body, is by Queen Serenity taking the starseed into her body, and 9 months later, giving her dearest person new life. The star seeds new body becomes her daughter which is why Princess Serenity does not have a father. Like the Virgin Mary, and she gave birth on the moon, which is why Usagi has a crescent moon mark. (Those born on the planet can have the insignia of their planet. Which explains Queen Serenity.

Only after Usagi’s sorrow an remembering the Moon, the Silver Millennium and her mother had she been able to become Sailor Moon, because the moon finally accepts her.

The fact that Queen Serenity’s soul is binded to Usagi’s is why she was never reborn and why her memories were stored into the Palace on the Moon Kingdom, so she can always be with her daughter, not just through their bond but through all the memories they got to share together.

Side head canon: Only the senshi, Mamoru, and Usagi can see the spirit of the moon, because to me, the computer program and memory act as a powerbase for Queen Serenity’s soul bound to Usagi, can be seen like a ghost. Usagi has her soul bonded to her, so she sees her, and since Mamoru and the senshi are bonded to Usagi, they see what she does and Chibiusa through bloodline.

The original intention behind the Silver Millennium was not just to unite the kingdoms in peace and harmony, that was a given of what she wanted, but was really because she wanted the senshi, the inner and the outer senshi to bond with Usagi, hoping the moon would eventually be able to open up to her daughter and become a true Sailor Senshi, and showed how special and important being a Sailor Senshi is by working with the Senshi. Giving them purposes, because perhaps they were lost a lot like Galaxia had been. She gifted them with castles, and they became Royal Status, and they each had a reason to be a Sailor Senshi hoping to woo the moon and create friends for her daughter.

Queen Serenity knew Usagi would become a greater than her in power and status not because she can see the future which is why it’s so vague. Only Queen Serenity knows Usagi would become a Sailor Senshi, and who Usagi had been in a past life before Queen Serenity gave her rebirth. 

So yeah, I’m done babbling now. That was fun^^ What do you think?