If they're going to have a romance subplot I would rather it be something that builds up naturally over time. Something that ends up being mutual between the two of them. Maybe something that builds up as they work together.

I'm not a big SonAmy fan personally but it's more the fact that in the games it was something that felt forced rather than it naturally developing. If they're able to organically build a romance between the two then I would be all for it. Otherwise just have them be friends.

I think Tails would be capable of holding his own in a spin off series. Tails had two solo games and the Adventure games showed Tails coming into his own. In Sonic Adventure 1 Tails disarmed a bomb set by Eggman to destroy Station Sqaure without Sonic's input. In Sonic Adventure 2 Tails went to Prison Island to bust Sonic out of prison. Also when he believed Sonic died he was ready to take on the mantle of hero. It's too bad Sega did him dirty in the later games.

To write Tails off as simply a sidekick and not capable of holding his own is folly.

So long as they don't make her into a creepy obsessed fangirl like she was in the earlier games and actually write her well I'm fine with Amy having a major role in the film.

Also there's nothing wrong with having a mostly male cast in anything just like there is nothing wrong with a mostly female cast. The important thing is that the story is good and the characters are well written.