Simply put, in a standard EP squad it should go something like this:
Vader: best of your speed. He has to be the fastest. He is the turbo charger to your tm manipulation.
EP: speed/protection/potency. He needs to be fast, and assuming you have his unique zetad then protection is his lifeline. Potency helps but isn't really necessary as his lead gives 35%.
Sion: speed & protection/health. And if you can a little potency. Some like crit damage triangle but if he does his dps falls to 0 and is useless so I want him to survive. He is the tank! Also he has to have his unique zeta to function properly.
Thrawn: speed. Simple.
DN: speed/protection. Speed equals more force drains and dispels. And annihilate.
My current speed lineup is : DV, Sion, EP, Thrawn, DN.