The Action Stage is broken into three parts.
Initial Quickcast Phase
Action Phase
Final Quickcast Phase
Once you have left the Action Phase, no further actions may be taken except the quickcast action. Now if you cast Rouse the Beast during your final activation, but before entering the Final Quickcast Phase, then you still can activate your creature.
Lastly one comment. There is a Rules Discussion section in the forums. Normally posting this kind of question there will get you a quicker and more thorough response and most likely more correct answer. Please post any responses to the Rules Discussion area so you can be sure to get the best answers.
