Everyone's experience differs a lot or a little and that's fine and expected.
In the opinion of experts in our field (and hence IMO) and IME, the scaleless versus scaled fish medicating differences are an urban myth and self-propagating misinformation even though it is even put on meds bottle labels
http://www.planetcatfish.com/forum/viewtopic.php?f=7&t=41623#p283736 (E.g., Racoll in this thread is Rupert A. Collins, a Ph.D. in Ichthyology.) Perhaps there are cases where the difference may be observed but these cases appear convoluted (more factors at play, hence hard to draw conclusions) and rare.
You would benefit also from reading the stickies in this forum for more on ich, salt, etc. written by trusted experts on MFK, including on salt, ich, etc.:
http://www.monsterfishkeepers.com/forums/forumdisplay.php?35-Freshwater-Diseases-and-Health-Issues I find these very helpful, especially when there is a disagreement between us, the common hobbyists.
I used to "observe" that my Doradidae, raphs and large ones, were intolerant of salt even at 1/3 of a medicating dose. This was corroborated by statements on some websites, e.g., Dr. Johnson's koi vet site which spoke of salt treatments and listed the spp that are sensitive to salt including raphs. Yet after years I've come to realize my Doradids were dying from an infection, not from salt. The ich infestation was simply concurrent with the bacterial infection. Primary or secondary, does not matter. I just wasn't treating my fish right or it was too late.