Good questions. It's nice to see a new person asking these questions so mistakes can be avoided.

There is really no need to change the water now. In a couple months, use RO water for 10% monthly water changes. A 20% water change changes things by 20%, or less meaning Alk will go down, but not by much because the RO water mixed with salt will still have a dKH of 10. It's not like you are adding saltwater with a dKH of 0/zero. I wouldn't bother with changing water now.
If you follow the method in the link below, you can have coral and fish within a week. I give away the live stuff you need to do it. Call/text me and come visit.
Aloha

808-345-1049
750 E Lakepoint Dr. #4V Murray