I've half dosed the tank due to the catfish (though the guy at the LFS said Britlenoses and Banjos should be fine fully dosed as they are bony plated rather than 'soft' like loaches, if anyone could confirm that for me?)
i was just scrolling down to say that, in this case, your LFS guy is quite right.
The risk with treating scaleless fish, is that they soak up some of it through their skin, and end up overdosing. Although BNs and banjos don't have scales, they do have 'scutes', which are like thick armour (corydoras are the same) and means they won't soak up any extra medication.
You can use a normal dosage for them.
I'm not sure about the garlic, I've never done it.