Definitely wait until you've throughly sterilized that rack before any of your animals go near it. When you say "pair" I'm reading this as breeding your original animals to each other, and the new snakes to each other inside this new rack? If that interpretation is correct then my answer is going to be about 3 months, because the new animals need to be quarantined in a separate area before coming into contact with your animals. Separate tubs in the same rack is not effective quarintine and would put all of your animals at risk. After quarintine I would wait until they're settled and eating in their new home before pairing, for some this might be days, for others weeks. Some snakes just stress more readily than others. Congrats on the new additions though!