Yes, that's the way to do it and yes, you need the installer code. The partition is a property of the panel, not AD2USB--which is essentially a keyboard. There are ways to reset the code but it's best that the alarm company give the code to you so everyone's on the same page. Partitions are typically used when you have two or even three areas that need different security requirements, for a given overall coverage environment. It is no more difficult for the monitoring company to monitor one partition or three. They are monitoring the panel state, not specific zones or partitions.
It's a fairly straight forward process to modify the panel. Lots of sequential button presses, but very doable. All the manuals are on the internet, and if you follow the step by step, it should not be too difficult.