All catfish are scaleless w/o exception (and 3/4 of all fish are catfish). They have varying degrees of sensitivity. RTCs are not what I'd think of as sensitive. Unless directly stated by the manufacturer "do not use on such and such fish", generally any med can be used at half dose. I've used a full dose of plenty of meds on my RTCs without adverse reactions.
As for melafix and pimafix as a natural cure for bacterial infections it cleared up my oscars case hith very nicely after trying all kinds of chemicals and additives for years after he developed it ( because regular water changes didn't do the trick)