You can spread your initial combustion onto the first 3 turtles if your timing is good. However I switched to frost for our kill, because we had a bad setup (3 meele, no caster haste) and it felt smoother for the tasks i was assigned to do (killing + kicking turtles, helping with aoe on little adds whenever possible = when turtles were down), so I don't really know how combustion lines up with later turtle spawns.
I think fire is fine however, if you have a somewhat better comp and your group doesn't need your aoe on the adds too much. Because fire aoe on the little adds sucks imo. Adds are way to short-lived to cleave sufficiently or to setup a combustion to spread. Fire dmg on turtles is fine though (3 target cleave yeah ).
Also make sure that someone in your group places a slow effect (hunter trap, druid vortex, ...) directly underneath Tortos right before the turtles spawn. That way the will stay together longer and you can cleave more efficiently.
Well, that was not a very fire-friendly boss. Spreading combustion on the first turtles made the opening of the fight quite decent. But from there on it was one long struggle trying to get any sort of good dmg in. The weakness of the class really shined through trying to do decent AOE dmg on the bats. After a few tries I just ended up with casting one flamestrike on CD and then got back to boss instead. Our shadow priest and elemental shaman however, found the fight to be "amazing"....
On that specific fight, I dont see any real use brigning in mages.