I have 5000 products on my site, all with unique descriptions. I have just taken on a new surf brand and will have 40 tee shirts with the same product descrition. The only thing that will change will be the name of the tee shirt. Is this ok since the rest of my pages and URL's(4,960) are unique?
Yes. The only problem you will have is that Google will see 39 of the pages as duplicate content and won't include them in relevant search results. But as far as I know the rest of your site won't be penalized.