This is what I was about to ask. I have no idea why the convention exists but in English people just don't ever say pepper first. It's always "salt and/or pepper". It's perfectly fine English to say it the other way round but nobody does so it would sound funny to most people.
So my question would be: in Gaidhlig does this convention exist but in reverse? Or is it fine either way and just happened to fall this way at random?
The mods don't accept my argument, but I think they should accept any answer that results from an attempt to put into good English. There is a much longer discussion on this page. We get a constant stream of people caught by this one, and being marked wrong when your only offence is trying to translate into good English is harmful to language learning.
I thought I had commented on the general pattern but I can't find it. It seems that the rule is always to put the shorter word first in English, thus men and women but ladies and gentlemen. Just try thinking of other examples. Even when the syllable count is the same, it seems that you can use vowel length or number of consonants to predict which goes first. I would actually disagree that it is 'perfectly fine English'. Just because no grammar book that I have seen mentions the rule does not mean that it is not a rule. As far as I am concerned, if nine out of ten first-language English speakers who expressed an opinion say it sounds wrong – then it is wrong.
I am not aware that other languages (including Gàidhlig) are nearly so fussy. The discussion on the other page gives the mods' opinion that either order of P and/or S is acceptable in Gàidhlig, and that is my experience too.