You do NOT need to adjust the pH of your RO/DI water. If using it for topoff, just add it directly. If using it for SW, just mix the salt mix in. If it's a quality mix, the pH will be in the right range and require no adjustment.
Technically, 100% pure RO/DI water doesn't have a pH. If you attempted to measure it, you might get a 7, but the reading would be pretty meaningless.
Instant Ocean will replace everything you need in your tank. The trick is keeping an eye on certain minerals between water changes, if you know that something in your system is using them up. The key minerals most people watch are calcium, alkalinity, and potentially magnesium. Those three are all used by corals and other calcerous life forms, such as coralline algae, snails, etc.
So, if your tank is stuffed with coral, monitor those. If it isn't, you're probably ok on nothing but frequent water changes. If you do need to add those elements, do some searching on here - books worth of material have been written on Ca and Alk supplementation.