IMHO, there's generally little reason to manipulate things such as your buffer size manually. In fact, it's probably what's causing your issues. Setting the buffer size sets the buffer property on the out object, and setting it can only be done once. So by setting the buffer size, you are implicitly committing your response.
For reference:
http://java.sun.com/products/jsp/tags/11/syntaxref118.html You should be able to do any number of includes, with or without associated controllers, without issue. If you don't write a single thing to your response, don't set any properties, etc etc, you can probably then forward. But if you're doing includes, there's probably little reason to also do a forward.
As an example, in Struts/Tiles you can associate a controller with a view and include the view result by referencing the controller. i.e. do an include of showUserInfo.do which forwards to userInfo.jsp and voila, the result of userInfo is included. You don't need to do the forward.
If you have a set of processes that need followed in order, forward your way through them rather than trying to include them. If you're trying to include the results of servlet1 and servlet2 before forwarding to sevlet3, instead try
servlet1 forward to servlet2 forward to servlet3.
This gives a much more loose coupling and allows each servlet to determine whether to continue the processing or redirect to an error page or handler.