You can iron something that is made of 100 % polyester but you want to use another fabric over the top of the polyester so that the polyester does not melt from the irons heat.
Polyester is made of plastic and will melt if you apply direct heat from a hot iron to it.
So you'll damage the 100 % polyester if you use the hot iron directly on the polyester.
But if you take some regular fabric and place it over the top of the polyester first then you can iron the polyester without damaging it.
However don't leave the iron too long on the polyester.
When washing polyester the temperature of the water for washing 100 percent polyester is 40 F degrees.
You can wash polyester in warm water but to prevent shrinkage of the polyester you should never wash the polyester in a water temperature of more than 60 F degrees.
The hotter the water when washing polyester the more chance you have at shrinking the polyester.
As long as you keep the water temperature of the washing machine cooler than 60 F degrees when washing the polyester you should have no issue of shrinkage.
However if you do want to shrink 100 percent polyester you can get the polyester wet and then dip the polyester in hot boiling water for a minute or so.
The boiling water will shrink the polyester if that is what you want.