i think technically "pareto" is an adjective (i.e. pareto chart).
I would probably say "pareto charts" if I were to use it in a plural sense. I could see someone using "paretoes" for short. I would prefer the spelling "paretoes" as a plural, but I can't say for sure how often it is used or what those who use them commonly use for the plural.
(A pareto chart is used to help identify the problems with a system or process that occur the most frequently, in order to work first on eliminating the most common problems. It's like applying the 80-20 rule.)