I'm confused because 'long' is supposedly one of the adjectives coming before the noun, but for some reason, I've always learned 'long pants' as 'pantalons longs'... a quick search on many translators yields the same. Could someone explain this rule?
It is actually kinda weird but you can say it in both way for those 2 examples :
De longs pantalons = Des pantalons longs
Same goes for the "serviette" example :
Des serviettes longues = De longues serviettes
But you could also say :
"Des longues serviettes" instead of "De longues serviettes"
Wether you use in one way or another depends on how you want to formulate your sentence and on wich one sound the most natural.