WOW. I didn't look at your sig the 1st go round. You have a lot of tanks. All fresh I see. Looks like you need a SW setup to complete things. I have 1 FW 40gal and 1 SW 30gal. I thought I had my hands full. lol But if I had 500+ gals I'd say "Whats another 90gal." Besides you can carry over some of the methods from FW. You already do WC and topoffs. So you're in a groove. Just keep it going to the otherside.
You don't need to adjust the pH of RO/DI water. Since it has virtually no ions to buffer it, it has no buffering ability itself and will immediately go to the pH of the salt water upon addition to the tank. Not only that, but pH readings of water that is deionized can be inaccurate, so don't even trust the pH measurement of your RO/DI water.
I never test my top off (RO/DI) water for anything besides TDS. It's supposed to be 99% pure H2O so what would be the point? I used to dump 3-4 gals into my 90gal tank / sump system. You wouldn't even see an effect from the temperature of the colder top off water. I now mix up kalk and use an ATO.