When I first started doing that I used aluminum screws.they were not too soft...in fact the fact that they were softer than regular screws was considered a plus.

Then Chotas came out so I started using sheet metal screws and they worked just fine ...couldn't tell any significant difference from the aluminum.

Never had any back out.

As far as length ...it depends on the construction of the boot some have a thicker original sole than others ....and on those I liked to have the screw bite into it.