I see nothing wrong with them talking to a Roc or a Purple Worm. Prolly wouldn't do them alot of good to talk to the worm, but they can try.
Technically a spellcaster could burn through his vitality points on just spells, but ideally they be more conservative, or have a high enough CON score to help them out. You're right though, the system is harder on sorcerers... I'm not sure what to do about that. It might be good to increase them to a d6 HD for this setting... I'll have to think about it. I'm also thinking something might have to be done about heal spells, I mean potentially the way it's set up, if a cleric or druid or such is trying to heal themselves, they could potentailly end up having it not be worth it.
I'll be gone for about a week soon and when I get back I'll prolly have a ton more ideas for this to post up, so that could help things along alot.