Actually, I think that Michael Wincott would be far, far superiour for Magus. He has probably one of the coolest voices in all of acting. He's... well, he's the prophet of Truth in Halo II; Guy of Gisbourne in Robin Hood Prince of Thieves; the man in black in the Three Musketeers; etc.
Just listen to this voice:
http://www.michaelwincott.org/Wavs/dorleacfirstday.wav
...
THAT'S MAGUS! THAT IS MAGUS! YES!
Perfect voice, Daniel. Absolutely perfect. He just needs some more malice and emotion, and he'll have it made.
As for the others...to be honest, I would actually like Lucien Dodge for Frog and Robo from that Chrono Cross Audio Drama. He is a wonderful voice actor and has a wide range, and he's probably the only one who could make a voice that sounds like a Frog, as he did for the audio drama, albiet unintentionally.
For Queen Zeal, you need someone who sounds quite different from Pat Carrol, wonderful voice though that may be. To roll with the theme of vaguely English sounding accents with Magus' voice actor, let's go for someone who has a romantic English accented voice--for those who watch Stargate: SG-1, think Sarah, the women who was eventually possessed by Osiris as an example--who can also go dangerously insane and angry.
A similiar voice for Schala, except with more subdued tones for the most part, unless she's speaking passionately about something, such as when she asks Crono and friends to stop her mother.
For Crono, you want something away from the horrible English dubbed anime voices and closer to something like....well, I can't think of an actual person, but for Crono I see a voice that is generally happy-sounding, but also quite strong. Lioncourt from the Audio Drama was good, but that voice made Crono seem a lot older. Perhaps something like that, but younger, more fitting his teenager status.
For Marle, we want something that can simulatenously sound like your average teenager and a royal Princess, though usually the former. Unfortunately, no ideas come to mind.
Lucca is definitely not going to be voiced by Mandy Moore if I have anything to say about it. She needs a voice that sounds like the female singer from the Takerocker's Chrono audio doujin. If that means bringing in a Japanese women who is also fluent in English, so be it.
Ayla now...I have no idea, I'm sorry.