Quick Questions about Monsters as Characters

[Deaxsa]

1. If i wanted to play a monster, say a Succubus (http://www.d20srd.org/srd/monsters/demon.htm#succubus), would i have to start with 6 RHD? Nothing is very clear on whether or not i have to start with 6, just that I cannot start with 0 RHD and some class levels.

2. if i wanted to play as a monster with spellcasting as a class (say a Nymph (http://www.d20srd.org/srd/monsters/nymph.htm)), would changing my HD (either through leveling up and taking RHD, or by dying/being resurrected, or any number of other ways), would i cast spells as if i had lost/gained spells in that class (druid for Nymph)?

3. do innate spellcasting abilities always stack with the class that it is emulating, or does it need to explicitly say so as per the Rakshasa (http://www.d20srd.org/srd/monsters/rakshasa.htm)?

4. what class would a Couatl's (http://www.d20srd.org/srd/monsters/couatl.htm) spellcasting emulate?

[Treme]

UrPriest's handbook is probably a good place to get started

http://www.giantitp.com/forums/showthread.php?t=207928

[Urpriest]

UrPriest's handbook is probably a good place to get started

http://www.giantitp.com/forums/showthread.php?t=207928

This, yeah. To deal with specific points, though:

1. Yes, you have to start with 6RHD. All Succubi have at least 6RHD.

2. You can't take RHD on levelup unless you're a True Dragon. Some monsters explicitly have casting based on HD (Sylph, Phaerimm), but most don't. Nymphs just cast as a 7th level Druid no matter their HD.

3. They always stack, the section on associate class levels makes this explicit.

4. A Couatl explicitly casts as a Sorceror.