I did Spite 2-3 days ago with this set up....we had a warry taking the red beam, a rogue on blue and a 2nd rogue on green...rogue 1 took blue, rogue 2 took green and MT on red on phase 1. on the other phase 1 we had a fury warry taking red and the rogue exchanged beams..so the one that had blue before was on green and the rogue on green was on blue
I one-tank Kara in a pug every week (4 guildies + any random ppl we know or just happen to pick up). I always put alternate myself between red and green. I put whatever we brought along as "OT" (which basically means whoever letse the skeletal ushers on the way to Opera hit them) in the red/green as well. That's either a fury warr, arms warr, or sometimes a feral druid (rarely). That leaves the guild warlock for one blue phase and either a pugged s priest (if we have one) for the other blue phase, or a mage+something to share a blue phase.
For beam assignments, I almost always give precedence to people who have seen Spite and/or tanked a beam before over someone who has never seen it, if only because it's hard to accurately describe to someone that has never been involved in the fight.