[Sunred]

What we tend to consider 6th rangers are dubbed "tsuika senshi" or additional warriors in Japan. Other rangers/heroes after that are "bangai senshi" or extra warriors.

In the case of Beet and Stag I see them as the next exception since Go-on Wings in that the two of them are considered as one. So if Gai ever had to gokai change into Go-busters he probably would go Kamen Rider W with it and merge them together.

Post-Gokaiger here is how I look at it:

Go-Busters
Main: Red Buster, Blue Buster, Yellow Buster
Additional: Beet Buster, Stag Buster
Extra: None

Kyoryuger
Main: Kyoryu Red, Kyoryu Black, Kyoryu Blue, Kyoryu Green, Kyoryu Pink
Additional: Kyoryu Gold
Extra: Kyoryu Cyan, Kyoryu Grey, Kyoryu Violet, Kyoryu Silver, Kyoryu Navy

ToQger
Main: ToQ1, ToQ2, ToQ3, ToQ4, ToQ5
Additional: ToQ6
Extra: ToQ7

Ninninger
Main: Akaninger, Aoninger, Kininger, Shironinger, Momoninger
Additional: Starninger
Extra: Midoninger, Akaninger(Yoshitaka), Akaninger(Tsumuji)

Zyuohger
Main: Zyuoh Eagle/Gorilla/Whale, Zyouh Shark, Zyuoh Lion, Zyuoh Elephant, Zyuoh Tiger
Additional: Zyuoh TheWorld
Extra: Zyuoh Bird