Might be it possible to reduce the retrieval set of blog posts that generate the rss feed, via backend settings for a cut off limit? I know another user attempted to relay their concerns for this plugin in conjunction with very large data sets for blog posts, but even on a smaller scale, it might be nicer and more user friendly for a limitation of blog posts added to the generated xml.
I can understand wanting to add limitations, and a simple implementation like the one you have suggested is something I can see possibly adding in the future when I have the time. This plugin was meant to be a simple, free, bare-bones RSS feed. It was never intended to handle large, or even multiple blogs, not that I wouldn't mind seeing it handle that.
With that in mind, this is completely free and open source. The code can be found on GitHub and contributed to freely. The repository can be found here. In the mean-time I will look into solutions for your suggestion.