For GUMI I'd use Piggy uung a "g-3" flag or something like that. Piggy has a sweet voice like GUMI...You could also use Nizimine Kakoi!
For KAITO I'd use Rook, surely. Or then Otodamaya Nitaro with flags.
For MEIKO, Homurane Ray.
For Rin I'd use Haruka Nana or Devy-chan...Or then Raine Rena with some flags...But I think the best option would be Honoka Ellie or Tohoku Kiritan..
For Len Raine Reizo with some flags as well...
For Gakupo, Bullard...or Aido Oshiame Mature Bank...or Hibiki Shinji.
For Megurine Luka...maybe Akune Kin?
For Miku...Kasane Teto...Tohoku Itako is also kinda close imo...
For IA (ROCKS) Otodamya Ichihime using "g-3" flag...
That is all I could list XD some may not sound too close to the VOCALOID, but they are a sound-a-like voice