TDS meters need to be calibrated, just like electric pH meters do and refractometers for saltwater aquariums. That said, even with calibrated TDS meters (they are supposed to come pre-calibrated and need to be re-calibrated every now and then), you can still get different results from two different brands.
As you mentioned, your tap water may be 1-2 GH... which means that the TDS should be below 40, theoretically speaking. Of course, if you have high KH or other dissolved solids, this could mean a higher TDS from tap.
My question is... if you plan on switching to RO water once the tank is cycled, what do you plan on using to remineralize the water? Me personally, I would recommend exhausting the buffering substrate prior to adding inhabitants... especially shrimp. That said, the dragon rock may always skew your water parameters.