You ask some good but loaded questions, Jim. We tend to identify positions in basketball since it is easier to place players into identifiable roles that way (i.e. the PG runs the offense, the C gets rebounds and blocks shots). Yet, it is often never that easy. It also depends how you view positions too. I tend to look at positions based on who players are guarding, and that is why I don't call Ben Simmons a PG. He has the requisite skill to fill that role but he is also 6'10" 240 pounds. No one should want him chasing around Steph Curry or Russell Westbrook full time for instance.
But I think Simmons brings a lot of defensive versatility to go with his elite rebounding. I think that once he gets comfortable in defensive sets, that Simmons should be able to switch on a variety of guys in the league. This should make him a very valuable pick and roll defender since switching is such an important component of team defense in the NBA. But when it comes to where he fits best as a starting point, I think putting him on guys who are perimeter based players on a normal basis will stunt his defensive rebounding, and make it less likely he gets boards on that end. That would take away his effectiveness.
Mind, I wouldn't use PSU's Ed teams as something that can work in the league. The athletes are too good in the NBA to rely on guards pinching to help with rebounds all the time. There are far too many good shooters out there in the league to tell Simmons to go get the ball no matter who he is guarding.