Ektachrome is the consumer version of Kodak slide film.
No reason why slide film shouldn't work in an FSU camera. If exposures are a little off due to inexact shutter speeds, etc., a bit of adjustment next time will take care of that.
I have some cartridges of Velvia in the freezer, which they usually say will make it last indefinitely. But short of that, I'd say that expired slide film is a risk. Remember that the film becomes the slides during processing, so there's no second chance with printing. But as has often been said, you can't beat slide film for color and reproduction use.
Since buying a digital SLR, I haven't used film in a while. But it's still there.