Schmitt , the geologist, was as skilled a PLT as any other test pilot.

I have to disagree with you on this point. Schmitt did undergo pilot training as part of his astronaut training and did quite well, but that's a long way from attaining the skills of a test pilot. Test pilots have to have a great deal of experience before undergoing specialized training. Schmitt had neither the training or experience of a test pilot.

As for the Apollo LM "Pilots", it appears the only one of them to actually get to control a LM in flight was on Bean Apollo 12 when they were in radio silence behind the moon. For the actual missions, the LM Pilots actually called out checklist items and instrument readings to the Mission Commanders who did all the flying. They were trained to fly the LM, they just didn't get to do it.

Back to a Mars mission, the key factor for choosing the crews will be the fact that they'll be on their own and beyond any possibility of help for months at a time. They have to be able to handle anything the mission throws at them. One thing is for certain - stuff will break during the mission. The crew must not only be well trained, they must be adaptable and innovative. It wouldn't hurt if they could be competitive on a show like Junkyard Wars, able to build anything out of anything. Prima donnas who are afraid to get their hands dirty need not apply.

Piloting and engineering skills will be required for the long cruise phases of the mission as well as for the landing. Since any real Mars mission will have months on the surface, they need good scientific skills. At a minimum, there should be a geologist and microbiologist in the crew. There may be the need for other scientific disciplines as well. There are a lot of opportunities for someone to get sick or injured on the mission, so a doctor would be a good idea. Perhaps the doctor could also be a trained pathologist and able to perform the biological aspects of the scientific mission.

The necessary skill sets should drive the crew size, not the other way around. It's possible that we would only get one shot at a Mars mission, so we have to be prepared to make the most of it.