Unlike child support, spousal support is often very difficult (if not impossible) to modify after the divorce is final, so getting it right the first time is a must!
Depends on how you word the agreement. Minimum child support is one thing. All of the extras like school tuition, tutoring, car insurance,etc is another thing. If the 2 people can agree that it is alimony and not child support, it is legal to do that. It can even be done for part of the property agreement. It is rare for a judge to go against a mediated agreement. You can even put into the agreement that child support will not be renegotiated until a certain time period or a certain condition occurs. That too is legal if both parties agree. Even state laws require a reason for modifying child support and I have never seen anyone get to go back the next day.