Please do not use any information given by a shop. They cannot be trusted to give accurate information.
Look at this website
https://www.seriouslyfish.com/knowledge-base/ It is written by ichthyologists who know what they are talking about. Go through your list and see if that website agrees with the shop - where they differ, Seriously Fish will be correct not the shop.
Black neon tetra 1 to 12 dH
Emperor tetra 1 to 12 dH. Too big for 54 litres and too active to be kept with gouramis
Endler 15 to 35 dH
Flame tetra 1 to 12 dH
Glowlight danio 1 to 12 dH
Pygmy cory 2 to 15 dH. Needs a group of at least 10.
X-ray tetra 2 to 20 dH
Zebra danio 5 to 20 dH. Needs cooler water and a very long tank because of their swimming behaviour. Too active for gouramis.
Honey gourami 2 to 15. Needs placid tank mates which rules out a number of fish in your list.
Asian rummy nose- do you mean Hemigramus rhodostomus or one of the species that look similar? Hemigramus rhodostomus 2 to 15 dH
As you can see, your shop's hardness ranges are too high. The figures I've typed are all from Seriously Fish, and these are more accurate than the shop's figures.
Yes, you can keep soft water fish if you mix your tap water with RO water. But you can't keep endlers or any other hard water fish with them. It's either/or in the same tank, not both. Or have 2 tanks.
There are other issues with your list.
Emperor tetras are too big for a 54 litre tank (they need at least 90 cm length) and they are active fish which can't be mixed with sedate fish.
Zebra danios are temperate fish not tropical; they need to be kept at cooler temperatures. And they need a tank much bigger than their size would suggest simply because they are such fast swimmers and need a length of at leat 100cm to be able to swim properly. This swimming will also stress sedate fish.
Glowlight danios need a bigger tank than 54 litres, at least 80 cm long.
You mentioned in a previous post that you do want honey gouramis. These are sedate fish and cannot be kept with active fish or they will be so stressed they'll get sick.