You don't have to ISO him, but it can be a safer choice for all the crabs involved. Mites are a worry as they are so much easier to deal with before you've added the crab to the main tank and expose him to others. Also, new crabs are usually stressed and hungry and thirsty, and can be easily overwhelmed by both a new environment and new friends. Keeping them by themselves for a few days to recover and change shells can lead to a very calm crab once introduction time comes.
Once you know what you want to do, giving the new crab a bath to remove any old smells will lead to smooth introductions, and you can just put him into the main tank when you have to watch them for a few hours. Introduction between PPs and Es are often a bit since they don't know how to communicate with each other, but it very rarely ever leads to actual violence.
I would also copy the others in saying that a species buddy is a good choice if you can do it.
We always recommend that every species in your tank has at least one other of it's kind as it tends to lead to more natural behaviour and calmer crabs.
