Your water is too soft for livebearers (swordtails, platies, mollies, guppies, Endlers). They will continue to struggle and weaken, and at some point develop other problems they would (should) normally be able to fight off but cannot because of the weakening, and they will not live to the normal lifespan, guaranteed.
Bentosi tetra,
Hyphessobrycon bentosi (the fish linked in post #14) is not an active swimmer but more sedate; the behaviour you describe is clearly due to there only being three of them, and they are under stress. They must have a good sized group, at least 9-10 (the linked profile explains why), and they will tend to remain somewhat together and in the mid-lower region of the tank among plants or wood/branches. Most (but not all) of the species in what Palmer termed the "Rosy clade" behave this way...if they are healthy and not stressed. For the better health and "happiness" of the fish you must get 6-7 more. And same for the neons. If we want a species, we need to know what it requires to be in the best health and then we must ensure we provide that. Nothing less is fair to the fish.
"Rainbowfish" covers dozens of different-behaving and differently-sized fish species. Some would work, some would not. Goourami could work, depending upon species...I would avoid the Dwarf due to incurable disease risk.
LFS said this wasn't a problem as Swords have been in similar conditions in store
Two issues here. First, we have all learned (usually the hard way, by ending up with dead fish) that store staff advice is frequently inaccurate and misleading. Always research a fish species before bringing them home, it is the only way to avoid harming/killing the fish. Don't worry, we all went through this, so we're here to help you avoid some of it.
Second, fish in store tanks are rarely in a good environment. The store hbopes to sell the fish fairly quickly, and the expense of providing separate tanks with preferred water parameters and environmental decor is just not realistic. But the fish are only there for days or a few weeks (they hope), and usually the damagee is not lasting. Once they are home, you are totally responsible for their life, and that requires knowing what they need and providing it.