As far as the dervish is concerned: although you might be able to make the case that a dervish's ability should extend to the double scimitar, you're aware that there's nothing in the RAW that commits one to this, right? Abilities that apply to the scimitar don't automatically apply to the double scimitar -- a character with Weapon Focus (scimitar) couldn't apply that feat to the double scimitar, even though doing so might seem intuitive.
But I do agree that finessable double scimitars is a perfectly reasonable house-rule, as long as you don't allow some of the Eberron material that makes double scimitars very powerful weapons. (I'm thinking of the Revenant Blade prestige class.)