Do the paints make the fabric stiff ?

It depends on which paints you use and how thickly you apply them. In my experience, painting with Inktense and Sun Dyes do not affect the drape of the fabric, unless you use a lot of fabric medium with the Inktense.

All of the acrylic paints that I've used will affect the drape of the fabric to a certain extent, unless you use a lot of water to thin them down, and that could affect the integrity of the paint film. I only use the acrylic paints for art or wall quilts where drape of the fabric isn't important.

Rob