Changing the StandardDesign is dangerous because it holds all of the defaults for templates. Unless you've overridden every 'standard' template then you may experience gaps.
It might be better if I try to explain what I'm trying to do, and have you suggest a solution, if you would.
For the company site, I have created several template overrides. One of these is an override for the forum class in line view mode. Now, I would like the product site to use the same override for the forum class in line view mode because it makes use of the same forum, but I would like to avoid having to specify it as an override for the product site as well.
Also, for all templates that do not exist in neither the product nor the company site, I would like a fallback on the standard site design.