RO water out of the tap, what are best products for conditioning and restoring kh?

I am unfamiliar with Kent RO Rite, but if it is for the same purpose as Equilibrium, then you can certainly use it instead.

PH by itself has small effect on fish when it swings, but if it swings combined with the change in water hardness then it could be deadly to fish if the change is too big and too quickly. If your water PH went from 6.4 to 7.6 in a few days (or even in a few hours), then it is no problem at all since the change is slow. By "too quickly", I mean in a few minutes. A slow PH change of 1~3 within a few hours is tolerable to most fish.

By the way, I don't think you need Neutral Regulator. According to the product description, it also adjust PH to around 7 which makes it a PH manipulator which I would avoid.

The bottom line, you do not need to fix PH for betta. Let it adapt on its own. What you need is to give the water mineral/buffer to make whatever PH it has to be fairly stable, so any change will be slow. (ex: without buffer, the PH can sudden crash from 8 to 3)