I've also got a Dervish build in the works. The one ability I struggle with is the Dervish's ability to wield a scimitar as a light weapon. I'd kind of want the character to use two scimitars in order gain any advantage from this ability. So might I suggest the same for you? It'd stack better with Improved Critical at 12th level anyway. Of course if you want to use a longsword for rp purposes, then that's cool too.

I figured I'd use a longsword in the main hand, and a scimitar in the off-hand. A tempest has the weapon versatility class feature, so feats like Weapon Focus and Improved Critical apply to both weapons when fighting with two weapons, even if they are different.

But you aren't the first person to tell me it'd be better to pick Weapon Focus and Improved Critical for scimitar, and even if I fought with a longsword in the main hand, it'd end up the same. So I think I'll do that.

That feature in Dervish bothers me. I don't like that the class encourages you to use a certain weapon that doesn't necessarily fit the class. Why require Weapon Focus(slashing) rather than Weapon Focus(scimitar) if you're going to give a bonus with a scimitar? Why not give a bonus with any slashing weapon ("Any 1-handed slashing weapon may be used as a light weapon"). It doesn't seem consistent.

Frankly I'm not a big fan of scimitars, but I'll use one to take advantage of the class feature..

What do you mean by bonus to scimitar? The only one I noticed was treating it as a light weapon. Everything else seems to mention slashing weapons.

That's what a meant. I guess bonus is actually a technically term; it doesn't give a bonus per se, but it does give a special ability that applies only to scimitars, rather than to all special weapons. Which doesn't sit well with me since the class doesn't suggest scimitars in any other way.

And this, my friend, is why we rewrite prestige classes. :-) (With the DM's approval, of course.)

I didn't scrutinize the build, but I love the concept!

For one, why barbarian? Most people I know whom take barbarian do it for one of two reasons. You listed those two reasons but some other classes I think would augment you character better. for one, skills are a fried concept there. while grey elves may have perform as a class skill all the time, they also take a hit on strength, so barbarian is just an attempt to make up for it. the bonus con and will saves is levied against a drop to ac (-2) which is quite contrary to most of what a dervish does really. For one you AC is dependant upon limited dodge bonuses and no armour heavier than light. so a subtraction is hard hitting especially since no use shield while dancing. next you cant rage while dancing, says it explicately. so you can only use one at a time. lastly, you cant use one after then other since both fatigue you, meaning after the second instance of use, god forbid your still in combat because your bumped down to exhausted from which there is no recover.
if your using complete warrior, try swashbuckler. good skills and HD, solid fighter progression for BAB and you get weapon finesses at 1st level. next, try samurai instead of ranger. i know you take a hit on skill points, but it gets you bastard sword proficiency, and nothing is quite as much fun as katana in one hand, scimitar in the other.
on another possibility is just chuck around the levels entirely. you have the first five to really work with since the next group is all full with tempest and dervish, so how about swashbuckler 3 fighter two? that gives you 4 optional feats, a bonus on light weapons from you INT which you can kick high since it will also build you skill points and I believe grey elves get a bonus to that, bonus weapon finesse and a small boost to reflex saves. go drizzt with dual scims?
or, try two of samurai two of fighter and level it out with one of swashbuckler. taking weapon finesse in mid to late level is kind of weird. Even if your starting the character at high levels, there logical survivability would be based on taking advantage of there best attributes early, it just seems fair to me.

I wanted a full BAB progression. That rules out rogue, bard, cleric, wizard, sorcerer, and monk. Not to mention that missing out on a single point of BAB would pretty much prevent me from either 1) taking the last level of dervish or 2) taking the last level of tempest.

Paladin did not fit my character concept. Ranger 3 and Fighter 3 don't add anything worthwhile. Barbarian has 4 skill points, and the speed enhancement stacks with the dervish's. Dervish dance, as great as it is, requires room to use, while rage doesn't.

We don't use swashbuckler. We don't use samurai. In which case, wasting a feat on an Exotic Weapon Proficiency for a weapon that can't be used with Weapon Finesse hardly seems worth it.