Mammillaria schiedeana plumosa

Are the red things fruits?

Yes, they're berries.

Will the seed be viable?

I don't know. I've never tried sewing them. Lots of my cacti get berries on them, but they usually shrivel up after a few weeks.

My "Easter Cactus" produced fruits that took almost a year to ripen, yet the seed started to germinate in just a few days. As a houseplant though it gets a constant temp all year, but I'm guessing that you keep many of your collection coolish over the winter?

I keep them in a heated conservatory during the winter, but it's on the cool side in there at night because of the glass.
I keep them dryish during winter too.