If you're looking at real soft focus lenses, as long as the condition isn't dreadful, you're okay. Meaning, a little separation around the edges is no big deal, but two elements that are completely separated so you can't really see through them would need to be cleaned and recemented to be useful.

My favorites are the Wollensak Verito and the Voigtländer Petzval. The Petzval isn't really a soft focus lens, but it's a nice portrait lens with a lot of curvature of field, so you can get some interesting pictorial effects. I also like the Voigtländer Heliar, as you can see from my avatar, but it's not a true soft focus lens either, though it produces an almost three dimensional separation between the sharp and unsharp parts of the negative, which is a great classic effect. The Universal Heliar is a true soft focus lens with soft focus control by means of a ring that moves the internal fifth element.

For 8x10" portraits, a lens of around 14-inches or 360mm is usually a good choice, but you can go longer if you have enough bellows, and you could go as short as maybe 10" or 250mm, if you're okay with a bit less working room, but it's not a dramatic difference. Because you're really in the macro range when making a portrait with 8x10" or larger, rules about using a lens that's twice the "normal" focal length for the format don't apply in the same way as they do for 35mm or medium format. Ron Wisner had a good article about this at one time, but I don't believe his website is up any more. It might be floating out there on some internet archive site or in the google cache.