1) Does it matter if the html and ascx files for the same skin are not the same?
>>>If someone happed to parse the skin again it will overwrite the ascx skin. I am assuming that the ascx skin is the latest.
2)Is the html file needed or can the skin packages be created with ascx files?
>>>>One can indeed create the skin as ascx files
3) When a new skin is installed via the DNN installer and the skin is the portal's default skin are additional resources needed to "update" the skin throughout the site - especially if the site is large (i.e. thousands of pages?) (therefore causing the site to slowdown)
>>>I dont fully follow the question. But it does not matter how many pages are in your site. If you have choosen a set of skin types and containers for your site via Admin/site settings/appearance >> then this will be reflected through out the site - unless you set a differnet skintype and container for the page in page functions which will overwrite the default settings.
4) Can a new default skin installation this cause slowdowns or errors if there are many users and many external visitors to the same portal?
No - possible errors on a skinpack is not related to number of users. It will show up if one or 1000s of users are looking at the page.
Salar