I'm using Silverstripe 3.0.3 and have this same issue on 2 separate sites I've tested. Content managers can only add new pages if the pagetype is Page. Any custom page that extends page leads to a "Forbidden" error. Here are a few screenshots to illustrate what I mean.
I think I've narrowed down the issue. Looks like it's tied to the "Who can create pages in the root of the site?" option. Once that is removed content managers can add any pagetype… When they do it's added to the root of the site despite properly selecting the "Under another page" option properly. Is this a known bug?