Great to hear you are in the process of completing your Capstone project! That’s a big step.
With regards to assignment questions, i think your teacher will have the most accurate response, so I would double check. Most often teachers are more than willing to answer assignment questions.
Another suggestion is to look over the assignments grading rubrics, they often have details regarding citations requirements and formatting.
In regards to your specific question, I believe that introducing a new source midway through a research proposal is technically fine, given that you cite the credited source correctly and include it in your Reference page. However, a new source might cause some confusion for the reader.
Ultimately I think double checking with your teacher might be most helpful.
Hope that helps!