In XF1.5 there is an option in the Admin CP which will allow you to change the number of posts displayed.
Search your Admin CP for "forumListNewPosts" and the option should come up.
For point 2 you'd either need to use an add-on such as "Widget Framework" or you'd need to upgrade to XF2 which will allow you to place the "New posts" widget anywhere with various options including which forums the widget pulls posts from.