Oh, I thought they had become normal characters starting with this star fest, hence my puzzlement. Thanks for the heads up.
If you have those two then either Riruru or Charlotta would fit best, IMO (I always forget about Silva). Do keep in mind, though, that Silva hits the damage cap a bit early, and thus might be unable to benefit from Riruru's buff - read a bit into this before you decide.
I think the other characters are actually really good, but don't really fit your team:
Sochie is good (Sochie and Yoda were my first water SSR) but seems to miss a lot, even her stall/slow skill. Her damage is ok, but nothing to write home about. Her heal actually seems to be pretty good if you can't get SSR Katlina, but I think she truly shines when you are using Superstar (debuff succes rate up with GW harp/resistance down) - do consider her if you are going to play SS.
Though if you need a dedicated healer (Sochie is more of a Dark Fencer wannabe) you could look into getting Lily.
Lancelot is (IMO) a better Silva, but considering you already have Silva, you might want to get him at a later date, filling your empty spot first - or run both of them, but I'm not so sure how it'd work. However, sicne Lancelot is human, he will benefit from the Bahamut dagger you might already have/will probably craft.
In the end, I think it's about personal preference, but I do think Charlotta and Riruru fit your team best, considering you have those two.