Actually, I'd cook hers first. Then let the bath cool down to your lower temp and drop the other steaks in.
The extra time won't make the well done steak any tougher - and keep them all warm, so you can finish them all together.
Also, the time - you really don't want to shorten your time to produce a "less done" version - that's what temperature's for. If anything, taking things out early will result in uneven cooking. Not a desirable result.
I'd likely be cooking them for an hour at least.
If you haven't seen them, the steak guide over on serious eats and the information (including thickness guidelines) over at Douglas Baldwin's site have a plethora of information on making the perfect steak.