The Orchard Dottyback will most likely either kill or cause the 6-Line to jump out of the tank. Dottybacks are pretty aggressive, even though the Orchard isn't as aggressive at some of the other ones, they're also very active and will most likely chase the 6-line around the tank.
A 90gal tank, though a good size reef tank, isn't that large of a tank for a fish like a Dottyback, which will cover the whole tank pretty easy.
In the end it's probably better to get rid of the 6-line and go with the Orchard Dottyback, as they usually causes less problems with other fish then the 6-line does.