I would strictly be using the incarnate has a shield. So I would think, even at it's worse, it would still take one hit away from the rest of the party correct? Maybe it would be okay if I make Summoning my secondary? For example if I had 6 in pyro, 3 in summon and 1 in poly, then scale it that way.
Yea it's perfectly fine to use Summoning as a support skill and not dump all your skill points into it. In fact certain summons require other skills to summon them, like you can summon a Fire Slug with Pyro + Summoning and an Artillery Plant with Geo + Summoning.
There's so many different builds that can work in this game. Also keep in mind as you progress you'll gain access to an item that allows you to redistribute skill points as necessary. So you could get rid of summoning if you don't want it anymore or go crazy and go pure summoning while using the other skills to just support it.
Incarnates get Provoke by default and can even help heal your party if you summon them from a water surface or use Water Infusion.
As far as polymorph, most abilities you're going to want don't require much investment in polymorph so feel free to just add 2-3 points to it and then ignore it.
Though keep in mind even 2 points in Scoundrel and/or Huntsman is helpful just for Cloak and Dagger and Tactical Retreat. Being able to teleport around the map for the low cost of 1 AP comes in handy a lot.