When is child support considered so be excessive, and how can a non custodial parent fight the excessive payments when he/she cannot afford legal representation; As well as is there a maximum amount that can be withheld from your income so that you as a person may be able to provide shelter for yourself as well as food and other essential items?

Child support is set by state laws depending on various incomes.
And most non custodial parents always think that their support is excessiver. And there is nothing you can do, it is set by forumla depending on your income. I will assume you had a hearing and provided them that info. If not you need to request ( make a motion) to have the child support modified due to a change in your income status)

I believe they will get 1/4 of your pay