I feel like on a 666 wave engineers will be REALLY difficult, because in normal games you at least have the option of just turtling up and taking the waves until the end in response to an engineer: it's all part of the normal process of being slowly pushed back. However, in 666 waves you can't be pushed back: you have to counter-push and regain ground between sub-waves, and engineers would make that really hard with sentries and teleporters.
They might work as a wave of their own (like five engineers (no sentries) and a couple of dozen easy bots) but I doubt they'd be a good support.