If you are going to be the only programmer on the project then your reasons make sense. However, if you are going to have other people on the projects - then your reasons do not make sense from a longer term point of view. How many lisp programmers do you have out there as compared to python or ruby programmers? If you and the founders decide to go your separate ways how likely is the project going to be continued in lisp?

It seems like a somewhat precarious decision to let you use Lisp for this project. There's nothing supremely special about the problem you are attacking that warrants Lisp (outside of your comfort level which you clarified very clearly in your post).

Perhaps I'm just old and cynical but I've been on too many projects where the first (and therefore "lead") developer chose a particular language and architecture that in the long term was not sustainable.