A matter of choice really, as said what is better for you OR whoever will maintain it.
My issue with multiple style sheets is interfierence. It is easy to create elements that interfier with others on a different sheet.
As you write it it sounds like you have pre-made style sheets for scripts you are using, so that is fine.
I always use one.
However while working on a high level web application for the state, we used many. As I recall it, we had one for basic HTML tags, then we had one for specific items like forms, and ones for specific pieces of the application, one for ASP wizzards and one for the login section... This was the decision of the lead programmer. I hated it at first but although I was technically in charge of the look and feel as the only real web guy... each of 4 programmers made their own pages for their part. In the end multiple external just worked better.
So I would say the scope of the project is to be considered. Small projects I do not bother. But massive ones are likley easier with multiple. of course I would have a mobile style sheet separate from the general one.