It is true that you team up with another person with the Grass type starter, if you chose the Fire one that is, but there is no main character with the Snivy starter Pokemon that plays with/against you in the story. You will have to trade with other people to get it or sometimes Snivy is used at the Battle Subway. Serperior, I'm not so sure.
Hope I help :D
Source- I have Black 2