"Hotaru's age fluctuates during the series; she is first introduced as a 12-year-old, is reborn as an infant, quickly develops into a young child around 4, 5, or 8 then reaches her proper chronological age after a metaphysical epiphany...Naoko Takeuchi, the series creator, describes her as delicate, quiet, precocious, and expressionless." -Sailor Saturn Wikipedia

Sailor Saturn's personality should also be taken with a grain of salt. I don't want her to sound nearly as child-like as Sailor Chibi Moon, but still be a young-sounding voice. 

You do not have to copy the performer's voices, but rather just mimic the essence of the character's voice. Sailor Moon should be cheerful and bright, Neptune should be calm and mature, etc.

Please use the mic you intend you complete this project with if you get the part. Have your audition be acapella, but in time with with the song. Use the video above to reference the song.