Deviation from guidelines is not based upon prior obligations for support. However, any prior support obligation is considered when calculating child support, so for example, the newer child's 20% would already take into consideration the prior child's support. Judges are usually not inclined to deviate much from guidelines without a strong argument in favor of that deviation. You really should have an attorney assist you with this.
If you have a split custody arrangement as you described, there is a good chance to have support deviated or possibly have an offsetting of child support based on the 20% the mother would pay you. Regarding the payment of the traffic tickets, I would not advise a client to not pay support, but clients need to make their own decisions as to what they should spend their money on. It may not be an issue if you pay back the unpaid support prior to the Mother taking you to court. Unless support is modified, you will need to pay the support and interest regardless, but it appears you may have a decent defense to any contempt proceedings. I strongly recommend you contact an attorney in your area to discuss this issue in more detail and get proper representation.
Any answers or information provided is for general information purposes only and is not intended to be a legal opinion, legal advice or a complete discussion of the legal issues. This is not intended to create a attorney-client relationship. Each individual's situation is different and you should seek independent legal advice from an attorney familiar with the laws of your state for specific information.
the traffic tickets are your problem, your doing, not a reason to reduce support. drive more carefully and you will not get tickets.
you most likely will get no deviation from the child supt. for the second child because of the first child since you are not paying support for the first child pursuant to a court order.
kids are expensive. they need to be supported. you chose to have two so close in age. if you do not support your children, who should?