Is it ok right ater mixing to put it in?

Instant Ocean is safe to add after mixing as long as you added dechlor. Add the salt slow and watch your hydrometer. Its a pain to add saltwater and drain and add freshwater, its a waste of salt too. You do not have fish yet, so it really doesn't matter right now, once you add your saltwater and when the salinity is in the green and your PH is good, wait 48 hours and add shrimp or fish (like you would eat ) to do a fishless cycle or add 3 yellow tail damsels. Test every 3 days for ammonia when ammonia reads 0 start testing for nitrite and so on to nitrate, and do 25% water change weekly or as needed. Add Bio Spira it will cycle your tank quickly.