I'm afraid that a GH of 10 is too low for mollies. They are fish which must have harder water. The best thing you can do is to rehome them. Your other fish like soft water so adding minerals to make the water harder is not an option.
Is the tank still the Lido 120? This is a tall short tank, measurements 61 cm (24 inches) wide x 58 cm (23 inches) tall x 41cm (16 inches) front to back. I'm afraid the tank is not long enough for even balloon mollies which need a tank 90 cm/36 inches long, adding to the problems for the mollies.
The Lido 120 is also too small for a rainbow shark, I'm afraid - this fish needs a tank with a footprint of 120 x 45 cm (48 x 18 inches)
To be honest, you need to rehome the mollies and the shark, then increase the numbers of harlequins and tetras. They should all be in groups of at least 6.
Can I ask, what are "dwarf mini honey gouramis"? There is only 1 species of honey gourami, though it does come in several colours, and none of them can be called dwarf or mini, only standard sized honey gouramis.