no of course not.. (otherwise would I suggest that?)
You can redirect the whole directory to an other directory. Besides even if it's file redirection it works with dynamic content. That is, if you redirect showthread.php it redirects all calls to this file including dynamic ones like "showthread.php?yourcontent here".
BTW why dont you move your forum to your maindirectory? This can be wise solution. If you dont want your members to lose bookmarks, you can redirect that directory to your new URL.