"Do you have a minute?"
Translation:Har du tid en minut?
I see, thanks for letting me know. I think what they said was that "har du/ni tid en minut" seems strange to them but that they have heard "har du/ni tid" a fair bit? But they have a lot more exposure to (and are far better at) written over spoken Swedish so maybe that's why they haven't come across it.
It's just a set phrase, they often don't make any sense if you try to interpret them literally.
Like the English phrase "all of a sudden".
Let's look at it sort of analytically:
What even is a (single) "sudden"?
What does "all of" refer to, we only have one thing (a "sudden")?
A "sudden" isn't even a thing!
TL;DR Set phrases don't have to make sense. It's just how it's said.
Most people in Sweden do not use ni as a polite pronoun, only for plural. The short version is that its' OK to use ni as a polite pronoun in Finland (but not necessary), but risky in Sweden – some people do use it that way, but the number of people who dislike that usage is much bigger, and some people dislike it intensely. There's a more detailed thread about this that you can find via the sticky under Discussions.
du is always for singular of course.