I think sometimes these aren't even indicative of how many Rangers there are. The Go-busters preview also had 6 black shapes, if I recall.

I don't think it'll be whistles. It'll be hard to use in a suit :p
I know we've had Dragonranger and his flute, but that was one Ranger with one item, not a central gimmick.

How well did Kiva work out?

Whistles were blown by Kivat :p
Also doesn't help that the Fuestles were pretty much dropped as a gimmick. They existed, but the only one ever used was Wake Up after, like, episode 15 :p

Oh well, you know I havent seen Kiva, and dont want to after the things Ive seen from it :lol

And not at all once he gained Tatsulot.

and Tatsulot.