Good idea, not so good outcome.
Before i say anything else i just wanna say that i am a taismo fan myself and that the last thing i wanna be known as is the mean person who gives a low rating to a flash that most others think deserves better.
Sorry to tell you this but you arn't that good at writing diolouge. I'm working on a non-flash fan-game of sonic x, mario and kingdom hearts and i know it can be hard to write diolouge that doesn't sound cheesy, or like something written by 4kids, but if you were to make a sequel of some kind, then maybe you could fix this problem.
Secondly, who said that the plant cosmo left behind was the reincarnation of her anyway? for all we know it could be her offspring or something. i don't mean to say that the plant being cosmo is a bad idea, it's just a bit odd having tails say "I forgot to water cosmo," at the very begining of the flash when he doesn't know what the plant will grow up to be. It was also a bit wierd how the plant turned into that monster. that plant is supposed to be cosmo remember!
The backgrounds wern't that bad, but green hill looked a bit... blank. some trees or something like that would have been a nice addition. i also thought that it was great that tails (my favorite sonic character) was portrayed as the hero, rather than the annoying little sidekick.
and in case you didn't know this, the original japanese version of sonic x is actually good, and if you actually like the english version, then you probably won't when you see it! if you're a taismo fan and you havn't seen the japanese version of episode 77, then you nedd to watch it on youtube NOW!!!
and lastly, tails can go into super form in sonic the hedgehog 3 on the sega megadrive, and when he does he is normally surounded by flickeys (the little birds that come out of eggman's robots when you destroy them). search "super tails" on deviant art or google images and you'll know what i mean.
if it wern't for these issues, and if the animation was just that little bit better, then i'm sure that this would have been a great flash.