The last time this issue came up, I recommended the following free e-book on software pricing, and I still stand by the recommendation: http://www.neildavidson.com/dontjustrollthedice.html. That should get you thinking about the right issues.

Now, as to your specific questions:

1) I'd start charging right away. If you can't get enough users to get good feedback, then that's feedback of a different kind. I don't see any serious upside to getting lots of free users, if you don't already have a business model built around that.

2) My gut is telling me that $7/month sounds low for this kind of product. If you've got compelling advantages over Balsamiq, and a good way to induce trial (i.e., the "free option" you mention), then I'd be thinking in the $15-20/month ballpark. I say this because as a business user, honestly, when evaluating a product that will help my business earn money more efficiently, the difference between $7/month and $20/month is basically a rounding error. They both fall into the "pocket change" bucket.