I know the forums are full of people saying 1 tank / 2 heal, but that is far from the only way to do this fight. We did it with 2 tanks / 3 healers to begin, and only recently switched to 2 tanks / 2 healers as we geared up. I have no real intention of switching to 1+2, I see it as a 'zerg strat' that lacks control. If you go for 2+3 then you have to actually handle the mechanics a lot better. That means coping with the fact that elementium bolts will land while the corruption is still slapping the platform: you will have to ensure that appropriate raid cooldowns are planned, that people are intelligently positioned (away from the bolt landing spot and away from one another before tentacle slaps), that you have discipline in your final phase dps (holding at 11% until after the 2nd round of terrors are dead).I know that we need to be single tanking, dual healing
As to your raid comp issues, maybe you should discuss whether the paly has considered a ret offspec? In 10 man raids, it's very rare that you decide "I want less tanks and more healers for this". Much more common to say "tank switch to dps". That said, if you look forward to heroic progression, you might well find that you want an arms warrior for fights like yorsahj and hagara anyway, so perhaps you need to find a way to encourage him to skill up. I have to wonder how well 'the tank from the other guild telling me how to play' goes down - you might want to try an indirect way to express that there is room for him to improve.