How can I add to a Supplemental Petition to Modify Custody?

file a motion for leave to amend the petition and attach the amended petition and set the matter for a hearing. 15 minute hearing.

Can you go to jail if both the mother and son are fighting and of age?

mutual combat means both sides committed a battery so potentially both can go to jail.

How to allow a letter from Psychologist be entered into evidence at custody trial without having testimony?

no

Am I allowed to depose my ex before our custody trial as we are both pro-se?

yes, she is a party to the case.

Is there any case law citing the definition of change in circumstance to modify alimony as "permanent in nature and foreseeable"

as his ability to pay goes, so does the award so if evidence points to a reduction of the award because of unforseen and unpredictable market...

I filed a counterpetition now my spouse wants to nonsuit from one state and file in another can he

If you filed a counter-petition then even if he dismisses his case your case is still open.

as a matter of law they can only bring up matters since the last custody order, not prior to that

What steps do I need to take in order to get the cheapest non contested divorce if I live in Florida and he lives in Illinois?

get the forms from the clerk or hire me my fee is $500

go back to family court, not small claims court.

If a temporary injunction is ordered can the petitioner harass the respondent?

It is not proper for the petition to do that. You can file a motion to dismiss the case if it continues