This workflow could be problematic, for a number of reasons. You may have interlacing going on in your FFs, you may need to deinterlace them. Regarding length, you can set the still duration in user prefs and then make a new project, the length will apply. Generally, FCP sucks for this.
When I use stills for a slideshow, I normally use Fotomagico, which is designed for this. last week someone here was griping about spending many, many hours doing a slideshow in FCP, in Fotomagico you do them in minutes.
Unfortunately, I don’t think you can use your FFs in it, but if you could, you can highlight all the stills and change all their durations with one mouse click.
Have a wonderful day.