You can do almost anything you wish on a "noindex" tagged page. You are telling the search engine bot to exclude the page from the search index, so the page should not affect your ranking.
The reason your site's # of pages is a factor, is your overall site is viewed as a whole. If you have a basic site with 10 pages, and there is a problem with 1 of the pages having duplicate content, then 10% of your site is affected, and this can impact how the search engine views your site. If your site hosted a forums with 10k pages, then that 1 page would represent 0.001 of your site, so the impact would not have any real effect.
