Definitely not Cilan. I don't hate Cilan, in fact he is one of my most favorite gym leaders, but Snivy is not really made for him.
For Gardenia, that gal prefers hardy grass-types. Tangrowth, Abomasnow, Torterra, even Victreebell is perfect for her. I just couldn't see her using a Snivy, Servine, or Serperior though. It's too... cute for her taste.
Erika, however, is the one for Snivy. She likes cute Pokemon, she is very caring, and very patient. Their personalities would complement (not match). It would be a perfect combination.