Your question kind of leads me to believe that you don’t know a whole lot about rabbit bonding?
A rabbit who gets along in a shelter is not ready to simply move in with your current rabbit, and they must be separated until they are bonded. Your rabbit can choose his friend (which will make bonding much easier) and then you bond them over a period of days (or months, it all depends) in neutral territory (bathrooms are popular), and then semi-neutral, until they are bonded.
Sometimes rabbits that get along just fine in the shelter just suddenly hate each other, even in neutral territory, when they get home, but this is kind of rare. https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/9/PID/940/Default.aspx
This is the BB section on bonding, which is a really good read if it’s your first time.
All rabbits do a little territorial marking. Some rabbits do go a little crazy when a new rabbit is introduced, this is popularly referred to as “poop wars”.