The majority is always N/2 + 1, where N is the number of members. A 6 member is less fault-tolerant than a 5 member cluster (quorum is 4 nodes instead of 3, and it still only allows for 2 nodes to fail).
The number of coordinators is separate from the number of boxes. You don't have to have a coordinator on every box.
I think you can set the number of coordinators to be even, but you never should - the fault tolerance will be strictly better if you decrease it by one.
This makes me wonder also what happens with multiple partitions, for example:
5 into 2, 2, and 1
7 into 3, 2, and 2
Or do you get around this by not deploying an uneven number of boxes?