Question about Red Mage casting

[argyleruby]

What kind of caster is a Red Mage?  Are they a Charisma-based caster where "casting modifier" is written in the description of a spell?  I know Charisma is the attribute against which bonus MP is based and what level of spell they can cast.  However, if a spell says effects/damage are based on Charisma OR Intelligence, which attribute modifier would you pick for a Red Mage?  And then, how does Ruby Knowledge work when calculating damage?
 
Here's an example scenario based on Aero with an Int of +2 and a Cha of +3 for a level 4 caster.

You direct a blast of wind at your opponent. Requires a ranged touch attack roll to hit that deals 1d6 points of wind damage + your casting modifier (Intelligence, Wisdom, or Charisma) + 1 point per caster level (maximum +5) and inflicts the Squalled status effect for 1d4 rounds. A successful Reflex save prevents the status effect from taking effect.

1d6 + 3 [Cha] + 4 [caster level] + 2 [Int for RN]
 
Or is it that because the Red Mage is a jack of all trades he has to use his Intelligence modifier, if he has one, in the damage calculation?  So he can heal, he can do damage, but he has to have high stats in multiple areas to be really effective.  
 
1d6 + 2[Int because of RN] +4[caster level]

And then I guess Ruby Knowledge Mastery can come into play which would increase the Int or Wis modifiers in the damage calculation if they have been chosen when Red Magery comes into effect.
 
There has been some disagreement in my group.

[Viladin]

What kind of caster is a Red Mage?  Are they a Charisma-based caster where "casting modifier" is written in the description of a spell?  I know Charisma is the attribute against which bonus MP is based and what level of spell they can cast.  However, if a spell says effects/damage are based on Charisma OR Intelligence, which attribute modifier would you pick for a Red Mage?  And then, how does Ruby Knowledge work when calculating damage?
 
Here's an example scenario based on Aero with an Int of +2 and a Cha of +3 for a level 4 caster.

You direct a blast of wind at your opponent. Requires a ranged touch attack roll to hit that deals 1d6 points of wind damage + your casting modifier (Intelligence, Wisdom, or Charisma) + 1 point per caster level (maximum +5) and inflicts the Squalled status effect for 1d4 rounds. A successful Reflex save prevents the status effect from taking effect.

1d6 + 3 [Cha] + 4 [caster level] + 2 [Int for RN]
 
Or is it that because the Red Mage is a jack of all trades he has to use his Intelligence modifier, if he has one, in the damage calculation?  So he can heal, he can do damage, but he has to have high stats in multiple areas to be really effective.  
 
1d6 + 2[Int because of RN] +4[caster level]

And then I guess Ruby Knowledge Mastery can come into play which would increase the Int or Wis modifiers in the damage calculation if they have been chosen when Red Magery comes into effect.
 
There has been some disagreement in my group.

Red Mage is Charisma-based caster. He uses his Charisma for any spell that opts to use Charisma. Ruby Knowledge allows the Red Mage to give a bonus to most damage spells with his Intelligence modifier or healing spells with his Wisdom modifier. Your first example is the correct one.