Im confused, do you have multiple tanks and one is hardwater and the other one you are going to soften with RO water ir do you have one tank that is hard and you are attempting to soften that?
If the latter mixing soft water fish like silver tip tetra( super nippy tetra) with guppies or platies that are hardwater fish either the platies will suffer or the tetras will suffer depending on if water is hard or soft. If its in the middle both will suffer as mollies and guppies and live bearers do better with 10dgh and above, higher even better and tetras do well with around 7 and lower. The water requirements for livebearers and tetras are on the complete opposite side of the dgh spectrum.
As for Scarlet Badis, they belong in species only tanks. They are shy terrotorial and picky eaters that will lose out for food as they are so small and slow picky eaters. Cool fish but shouldnt be kept with other fish.