RO water with high pH...why?

Because some CO2 gassed off and the water became thus more alkaline (with less carbonic acid).

 

You can get it neutral if you turn your kH down to zero. Adding HCl bring your kH down too, you might as well just do it through your RO unit. I don't think such small changes in pH matter, my tapwater has a pH of 7.8, I put it straight into my aquarium and my fish don't seem to mind. pH goes down over the week due to the high fishload and heavy feeding.

 

What's happening (possibly) is that your tap water pours out at 7.2 because of atmospheric CO2, where as the RO has had time to sit and gas off, so your tests are coming back higher pH despite lower KH. If you've got high TDS in your area, you may want to consider adding on a 4th stage to remove the larger solids, and a flush kit to prolong membrane life. There are some places (here for instance) where the TDS levels are so high that even 4 stage RO units blow their membranes within 6 months of regular use.

HCL is going to offer inconsistent pH reduction unless you use an auto doser or add it at least semi-regularly. You may be able to piggyback it in on your micros after an initial pre-dosing during WC's. HCL is already something used to prevent CSM+B in liquid mix from spoiling. Just theory here; I've only used HCL to sterilize and prevent mold in micros.

-Philosophos