Are you referring TOS or server performance?
I don't think it breaks any TOS.
If you are talking about server performance, it depends on the number of visits of your forum. It will be fine when you have a small number of visits at the beginning. However, the shared server will not be able to handle it when it grows because the resources are shared. By then, you should consider to switch to VPS to have independent resources for your forum. When VPS is not good enough for you, it is time for you to switch to dedicated server.
Hope this helps a bit.