Note: I have seen some massive Dumeril's in the lengths of 9-10' in captivity. They are very sweet and tame snakes, but they do not max out at 6-7'. Make sure you are aware!
Also, Nics are not terribly aggressive. I've worked with them in the past to help a pet shop out who was blind to their care, and they were absolutely non-agressive little worms. But when it comes to boa's I'm a Dumeril's fan all the way.