There is a cost in overcommitting as there is the opportunity cost. But I believe there is a way to overcommit with the "under commit, over deliver" line of thought. That will allow you to still look at the new opportunities and present an option ("sure - let's talk first"..."could get done by xx".."too late? well next time perhaps").

One thing I've found - while handling many side-projects is to know each one has a priority based on how much I'm invested in one of them and the benefits that one will bring. If you have the priorities set upfront, it'll be easier to make decisions in case something gotta give. But if you "under commit" to a lot of different commitments, you'd still have some space :)