Ironically, the first one was the baby brother.

Though he was basically Geki except more badass, so go figure.

I'm pretty sure all of the rangers that Gokai Silver uses and that are shown on his Gold Mode armor are considered as sixth rangers. In addition to those and Gokai Silver himself, Kyoryu Gold and ToQ6 are recognized as sixth rangers.

Bullblack Hyuuga, Signalman, Ninjaman, and Beast-Demon Hunter Zeek are debatable, but personally I wouldn't consider any of them as sixth rangers since Signalman and Ninjaman don't henshin from a humanoid form into their standard forms, and Beast-Demon Hunter Zeek only made an appearance in one episode. As for Hyuuga, since Bullblack is considered as an Extra Hero, it makes sense for Hyuuga to be considered as an Extra Hero as well. Everyone else like VRV Master, Deka Swan, Wolzard Fire, etc. are considered as Extra Heroes.

I consider Black Knight to be the 6th ranger of Gingaman, since they tend to fill the requirements most people set in place. However, since those "Requirements" are just fan made rules, officially, they hold no real precedence.

There is a precedence, Gai's shiny golden chest.

True, but Gokaiger also treated Black Knight a little differently. He was used with the Bonuses, but he also was saved in their first episode of their appearance while all the others weren't for a far longer time. It seemed like they purposefully treated him as a middle-ground between Sixth and Bonus.

Poor Signalman.

So because of that, AbareKiller is a sixth ranger, yet GekiChoppe isnt? Even though both are the fifth members of their team, and complete different motifs then the rest of team, along with their own megazords?

Abaranger technically had an unofficial fifth ranger, Abare Pink.

http://img4.wikia.nocookie.net/__cb2...Abare-Pink.jpg

Theres always a reason...

Yes... that's life I suppose. :(