In the past I have not made much effort to worry about scale with my modeling attempts in Houdini. However it has been brought to my attention that this isn't a very good practice to do. So, if you have an existing model that needs to be at a certain level of scale, is it acceptable to just add on a transform SOP to the geometry network to scale it to the correct size and then go on modeling from there?

Personally, I don't see why not.