Generally speaking, over-mixing isn't nearly as worrisome for gluten-free as for gluten containing flours. It is important for the dough to be mixed long enough to be visually lump free. It is generally better to mix at a lower speed, as this will provide the gentlest treatment of the flour grains. We like to mix breads a little longer to allow the gums and proteins to bond better. This also seems to help the yeast get fully and evenly dispersed.