This is a technical forum, and you will not get any authoritative answers here about legal matters.
My gut feeling is that you are good, but my knowledge is not any better than yours. You would need to talk to a Microsoft licensing specialist to get an answer you can trust.
But depending on what features you are planning to work with, you could let it suffice with Express Edition, which also is free, but which is licensed for Production. On the other hand, there is an upper limit of the database size of 10 GB, and it cannot use more than 1GB of RAM. Also, it does not support all features, but about everything that is exposed in the programmability area is available.