People keep koi ponds at a TDS of 30-40 Sam so your chocolate gouramis won't mind. Koi keepers even make sure that most of that 40ppm is made up from KH, which you won't need so your water will be even less 'pure' than their's in some way! If you add peat or any ferts to the water the TDS soon shoots up, even food adds to it!
I am surprised your output water has such a high TDS though from the membrane. Mine comes out with an undetectable TDS, with no DI stage. What's your input water like?
Right cheers for the info Ed I've had a bit of a rethink, was gonna just have choc's in the tank but the green neons currently in there look pretty good so may need a bit more TDS anyway. Hell the Choc are currently in a mid hardness tank and are doing great. Will see.
My tap is from the Thames region so it pretty hard at 280ish TDS.