If my opponent took all the combat damage from my {Rakdos, Lord of Riots} can I cast a {Wurmcoil Engine} for zero.
yes
Yes. As far as my knowledge goes. Rakdos reduces it by 1 per every 1 pt of damage... So 6 PTs = -{6}
Awesome thanks. I think I can do a big creature 5 color reanimator deck lol