AFAIK, Hillstream Loaches don't grow particularly big, so should be OK in the a 40L tank for sure.
As to LFS knowing about things: If you find one that DOES know more than you on more than a few things, and are honest when they DON'T know (which is actually MORE important...), then I'd recommend to stick with them. There are MANY fish shops that are staffed mainly by people who don't know much about anything. Usually there is SOMEONE there that actually knows at least a bit more, but not always. And finding this SOMEONE is often a problem, because they tend to be busy away from the shop floor, off on sat/sun, etc.
Salt is a good medication on certain conditions (ich for example), simply because it's more poisonous to the cause of the condition than to the fish. However, different fish are more or less tolerant to salt themselves, and what would kill some pest in the tank, would potentially also kill some fish. Some catfishes are less tolerant than many other fish, but there are many other fish that have a low tolerance too. Other catfish are quite tolerant to salt and some even require it, as they live as adults in brackish estuaries.
The general idea of adding salt (of any kind) to the tank is probably one of those propagated urban myths of the aquarium world. There was another thread about it on this forum during the weekend. The actual goodness/badness of it will depend very much on what fish you have, but unless it's specifically stated for that fish that they require salt, I wouldn't add any... [unless for medical purposes, and then for a specific illness that is known to be curable with salt, and only to fish that are actually affected].
I agree, some of the anticaking agents/additives can be very bad for fish, and I don't necessarily trust the labeling either. But I was using the term "table salt" loosely. Maybe I should have used "sodium chloride" instead... This to differentiate from say magnesium sulphate or potassium permanganate, which are also a salts (and not necessarily something you want to have in a fish-tank either).
And further apologies for the oversimplification on goldfish. I've got some sort of orange/white fantail myself [well, technically, it's fiance's, but I take care of the tank it's in...], and it's growing nicely, so I'm obviously not waving too much ammonia near it...
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Mats
I'm from the UK and have a 180L tank with four fancy goldfish and two bristlenose's, and more recently fell for the charms of a tiny butterfly plec who is absolutely gorgeous, he's now firmly installed a 40L tank. Both tanks are heated to 74 degrees.
I'll attempt to post a photo asap.
