There are some inverts that like 6 dGH, so lowering GH isn't necessarily a good thing, unless they receive all they need from diet.
TDS is total dissolved solids. It measures minerals, salts, waste, etc. Basically, everything in water that is organic or inorganic in the water column.
RO water should have between 5-7 pH, and 0 on everything else, except maybe TDS. It can show low TDS but still be RO. Low as in, preferably under 15 TDS. You might say that RO water is distilled water, but distilled water isn't always RO water.
TDS is less important than GH and KH, in the sense that it's possible to have high TDS and low GH or KH, or "low" TDS but higher KH and GH. TDS more or less measures how "clean" the water is. The lower the TDS, the cleaner the water is. The higher it is, the dirtier the water is. (it also measures fertilizers, too...) With inverts, at least shrimp, there is a target range of TDS to go for, although that would depend on the type of shrimp you have.