When my guild downed 10man Madness, we two healed it with myself and a holy priest. It's definitely two healable. The fight can get really close without a 6th dps.
It's really tough on mana, especially at the end, during the head phase. But coordinate your cooldowns and efficiently use your mana and you can pull through just fine.
The order my guild did it was Red -> Green -> Yellow -> Blue, which seemed to work pretty effectively.
Something I thought was worth noting; we used Heroism/BL/Timewarp on the first platform as soon as the corruption in the back spawned, and by the time we reached the head phase we would have it again, because the fight lasts so long. Because of this, when we reached the 4th platform without hero we didn't have enough dps to kill the corruption before the elementium bolt was fired, and so our paladin tank continued to tank the tentacle, and bubbled on impact of the bolt to survive. Because the corruption was still up, there was a threat of people getting slammed and killed because of the bolt doing so much heavy raid damage. For this I tried my best to save both tree form and tranq to use simultaneously in order to get everyone topped in as little time as absolutely possible, this seemed to work very effectively in countering any fatalities from a tentacle smash that followed the bolt.