Jan Kandou.

Do you guys think they should rush bringing in the 6th ranger of the group?

Nope.
Sixth ranger is to break status quo and keep things interesting.
Kind of a dumb thing to rush into a series, despite fans expecting it.

Plus it would completely mess up the release schedule for the merchandise.

Kenta I feel like it could be really chill to hang out with him

Mike (Mega Pink) because she makes me feel some kind of way

Nossan (Kyoryu Blue) and Ryuji (Blue Buster) just for the old man jokes they would tell (still think they should have their own mini series)

Akira (ToQ-6) just because

Luka (Gokai Yellow) for the same reason as Miku

I think if it is rushed, it will distract from the story. Utchy's early appearance worked with what they were doing for the overall season, but if they are brought in just because, then it would be dumb.

Out of all of the ninningers who haven't we get to know more of?

I think yakumo and to a degree takaharu also starninger but he hasn't appeared yet so he's not counted.

Moune. Completely Moune.

Why wasn't Gekiranger as well received in Japan? I know Japanese kids aren't as keen on villains are American children are, but I don't think that is really the reason why...

I have noticed that the more minimalistic/plot rather than toy-driven shows are not as well received in Japan. Gekiranger was pretty bare bones on the toy aspect (not as bad as Go-Busters though).