Hi there!
1 – The best would probably be to change 5-10L a week. Less is fine, more would not be good.
2 – They’ll happily eat frozen food. Some specimens even accept dried granules, altough that’s highly exceptional. Paros will eat anything from artemia nauplii to big fat mosquito larvae, if it moves, it’s food.
3 – I have no idea about TDS, but I try to keep my tanks below 50µS. My RO-unit gives 6-10µS, and my extremely soft tap water around is like 200µS, for reference. This is not really an absolute value, since they’ll also survive in tap water. In fact, Parosphromenus are very hardy fish, when given a tank of their own. They won’t breed, however, in tap water. Is has to be very soft and acidic for the eggs to develop and hatch.