ksmom3,
I can't say I blame you at all for wanting to establish a modified child support order. Things have definitely changed over 10 years. But, whether the extra monthly amount will be worth the legal expense and possible legal hassle is a question you have to ask yourself. I think most attorneys would probably charge you about $1000 to do it.
I'll tell you what I think I know - if your child support order was established before this other one you mentioned, yours takes preference as far as I am aware. So his full income would be used to establish the support for your child, then when he files for his second child, the child support (monthly) would be taken out of that in the income section of the guidelines. It sounds as if you have a copy of his other support order. If so, you can probably just use that income without any better number to look at.
As far as income for seasonal and periodic payments, the courts sometimes just look at average incomes. For your hourly job, they would probably look at your last few paychecks and determine the monthly average. With your commission checks, they'd probably do the same. They might look at the last 6 months average and project you normally receive a certain amount/month. If you sometimes don't get a commission check, you should mention that. These are treated just like performance bonuses and the courts are really trying to get a piece of that money.
If you try out the calculator provided on this site you will actually find out that your income will make very little difference in the final child support number. It is based about 95% off of his income. So, don't get too worried about your income levels. Worry more about proving his income and learn more about this other child support case. Since yours was filed first it should work in your favor.
If you need assistance filling out the worksheet, you could PM me your information and I'll send you a filled in sheet so you can see how it works.