@Aelin59 - question #1. How big is your pond? 13 koi will require a large volume of water.
As for quarantine - you'll get varying opinions. Some would say you should set up a separate holding tank and keep the 10 new fish in that well filtered tank for a period of weeks. Some do two weeks, some practice six week quarantine. Others - like me - will tell you to choose your supplier well and you will be bringing home already healthy fish. We've never quarantined fish - HOWEVER, we have rarely added any new fish since we opened our pond. We got them all at one time from one supplier and added them all at once.
However, the concern is not just protecting the original fish from the new fish; the new fish can also be susceptible to things that are already in the pond that they other fish are not bothered by. Or the process of transfer can weaken the new fish and make them more vulnerable to parasites or bacteria in the water that otherwise would not be a problem. Quarantine can be tricky business if you aren't set up to care for them in a healthy environment - and then they adjust to quarantine and you have to move them again. Those are things you may not find out until you finally do put the new fish into the pond.
Long story short - adding new fish has risks, no matter what method you choose.
Back to my original question - how big is your pond?