Your aquarium guy is 100% right on the money, read the chemistry link in my sig. RO by definition has almost no ionic strength and therefore its pH is of no consequence.

Tea is a broad term. There are beneficials packaged for hydro use- SubCulture comes to mind- that are quite happy in a solution containing calmag and pH adjusters. It's all about how MUCH is in there. Like, in the ocean, it's really salty but microorganisms of all sorts have adapted. Teas are useful for more than just the bennies too. Organic acids especially humic are found in compost teas, and those are great for stabilizing pH and making nutes available. If you have any doubts as to whether your bennies are suffering from addition of up/down, use a hydro beneifical product that you know will not be damaged. I can't tell you too much more than that because I don't run teas per se and haven't bothered to do much experimenting with them. But after you mix your nute solution I would still bump the pH up just a hair to get it closer to where the coco-based soil/soilless mix you are running needs.
I gotta run... sorry...