As a non-native English speaker I struggle with Greek prefixes. Am I allowed to use just normal English numbers in place of them? Is it natural? Or do I have to learn how those Greek prefixes work at the end of the day?
Mathematicians do use this form for bigger numbers. The Wikipedia article Heptadecagon currently contains the phrase “a regular 51-gon, 85-gon or 255-gon and any regular n-gon with 2h times as many sides”.
And in that context, you may find mathematicians using the form for smaller numbers: in an article about polygons of different sizes, I would not be surprised to meet “5-gon” or “8-gon”.
But outside mathematics, I’ve never heard anybody say “eight-gon” or any of the others.