His real challenge is not the plant, he will do just fine. As well as he will in adjusting to the boat's routines and expectations.
The one area that would concern me were I he, is if he is given the LPO assignment. For a just recently promoted E-6, he probably has had no divisional leadership responsibility. All to often, a good engineer does not make a good LPO. This is especially true for a guy with no sea time. For his sake, I hope he has time to grow into an LPO slot.
As far a qualifying goes, other then operational considerations, I knew a couple of guys who qualified as RO's on S3G at prototype, then D2G, and then A1W, all in a four and half year period. The time to qualify was 4-6 months. When you think about it, you could not find a more wildly different engineering environment then these three configurations. Oh, back in the day, SRO was a part of RO qualification, at least it was on my ship. SRO was a terrible watch, 8 hours of total boredom. The graveyard shift was the worse, just you, an MM somewhere in the engine room, and a roving EM you never saw.
Anyway, do not worry, he will do well. The real challenge is yours and how well you handle his sea time.
Good luck to the both of you.
Oh, no matter what is said, E-6 in four is something to be proud about.