I don't know about Silo, but I think most modellers will be consistent in the way they apply subdivision - meaning you can alter your low resolution model, subdivide and it will be usable as a morph. If subdivision changes vertex order, it won't work. Try it on a simple test piece and see. The thing with seams and creases is to have enough polygons, since Poser will over-smooth things and what looks acceptable in your modeller won't necessarily work in Poser. Take a look at the screen shot, showing where the upper of a boot meets the sole. Try to make your seam "valleys" trapeziodal in cross-section: a polygon for each slope, and a polygon for the base (the two I have here is overkill, and is due to the way I made the model). Don't make a V-shaped seam, because Poser will flatten it out. You don't usually need a complete second layer, unless it's something like a coat which may be opened to show the lining. To get thickness you can usually "turn over" the edge so there's a row of polygons on the inside of the model, which is enough to give the impression you want. Too many polys will inflate your file sizes, bog Poser down and make morphs harder to make.