Ive found cast sequences work just fine with casters... and the real reason to level a group of multiple kinds is just to get as many 70s as possible before wrath to give yourself some flexibility when it hits. I will agree that using multiple classes doesn't synergize as smoothly, and I'd definitely stay away from shadowpriests because they have no spammable ability. I've definitely used an instant cast instant cast long cast matching cast sequence for my lock/priest.