Not all payment plans are the same
Smaller balances can often be set up with little or no financial disclosure, on a simplified basis that mostly asks how much you owe and how quickly you will clear it. Larger balances, and cases where the payment needs to be lower than the straightforward arithmetic, require you to open your finances to the IRS and justify the figure.
Within that second group there is real room to negotiate. What the IRS allows for housing, transport, healthcare, and other living costs follows published standards, and knowing where those standards do and do not bind is most of the work.

