I personally don't think that Gekiranger did a good job with any of its heroes that weren't Jan. But then Gekiranger gave a lot of its focus to Rio and Mele, whereas (for instance) Liveman and Abaranger started with three and all were pretty well-drawn characters
It seems to me like three-man teams that introduce their additional Rangers at the same time as the main cast and then have them join the team later (Abaranger, Hurricaneger) do a better job with it than those that introduce two or three Rangers as new characters halfway through (Gekiranger, Liveman.) But it could just be a quirk of those shows' writing rather than how the characters are introduced. We'll see ...
Did Gekiranger do that badly? I know the toy sales were poor, but I was under the impression the show itself was quite popular. Go-onger is pretty much the opposite, having pretty mediocre ratings but it brought in more money from toys than any of the other seasons.
As for the 3 man vs 5 man thing, I think the 3 man start is a good idea. It does give them more chance to flesh out the characters a lot more before introducing more.. True, Gekiviolet and Chopper were handled very poorly, but that's because it seems their development and episodes that could have been spent on their stories were given to Rio and Mele, if those two were the extra's, it would have all meshed together a lot better.
I'm not sure if anyone has pointed it out yet, but we're not seeing a release of Buster Yellow's mecha until the end of March, meaning Gobuster-oh probably won't be appearing until quite a bit into the show. I think it's safe to say that we definitely won't be seeing the two additional guys joining within the first few episodes, like what happened with Hurricanger and Go-onger.