Now regarding not just Battle Nizer kaiju, but any kaiju, and also seijin (because let's face it if a certain Gomora get's it's own page, an alien can't be that far behind), that could be considered as a character, like how Toto(Wizard of Oz) is recognized even though he is just a dog, e.g for the Ultra Series, the Pigmon from Ultraman Zero, he became Ultimate Force Zero's unofficial mascot, and Zero also named him Moroboshi.
But then there's the question if we really need to give individual pages to such kaiju, in Rei's Gomora's case, he had a long history in the Ultra Galaxy series, as well as appearing along side the Ultramen in gaidens, Rei's Litra also appeared in every thing Gomora did, but aside from the first show and N.E.O she only appeared for one scene in the Ultra Galaxy Legends movie and the gaidens that Rei appeared in. So then Rei's Litra doesn't have as big of a history like Rei's Gomora. But should we still let her her have a page? Is it necessary? But what does having her own page do that can't be done on the Litra page? Well one could say that by giving her own page shows her significance to Rei, and how separate she is from just being "another Litra". Does that seem like a good enough reason to give her her own page? If yes than we seriously have to consider all other Battle Nizer Kaiju, Rei's Eleking, Kate's Gan Q, even the Reionyx Nackle's Galberos. However this might not mean every single Battle Nizer Kaiju should have their own page, as some were there for just one battle. Maybe, they should be listed on part of their master's page, which brings me to my next question.
As for aliens I think we need to really recognize what make a character significant, like the new Alien Fanton, Dr. Guruman from Ultraman X. I think he should have his own page, no question because he is a member of Xio and an obvious character in the show, and he is more obvious than Rei's Gomora. This is because since he is an alien he really just a man, from space. And many enemy seijin with developed personalities like Guruman that have appeared through out the Ultra Series but for pretty much only one episode each time.
A show with the best examples is Ultraseven, as many different groups of aliens that were all treated as individual characters. For example the only seen Alien Pegassa, he was an agent sent to Earth to see if anything could be done to avoid a collision between Earth and Pegassa city, and during his mission he encountered and befriended Dan and Anne, making him not want to hurt them when he thought he had destroy the Earth to avoid collision, that is character development so he was more than just an Alien Pegassa, he was a person.
So in light of all this I think certain alien members deserve their own pages, but we should research which ones case by case, I believe would make more use of the antagonist infobox since most of them would be characters with bad intentions.