To me this just feels like, "Hey there are these neat tools out there, I should go build a project using them," rather than actually needing to use these specific tools. Maybe I'm just being crabby but it feels like a classic case of premature optimization. You're trying to use an external service that costs money for an app that isn't even written yet and you don't say you've got a guaranteed audience or one that will necessarily scale to a level that warrants that.
"The application will (if successful) need to be very scalable in terms of # of users supported", seriously, that describes half the Internet. It's the "if successful" part that's really the question. Just concentrate on building the application quickly and easily. The easiest way to do that is just use ROR as it is out-of-the-box so to speak. Pair it with a database, use ActiveRecord and get something built and attracting users.
In fact, I'll go further and say that EC2 is rather expensive for always on servers. Deploy it over on Slicehost or another hosted solution and then move it to EC2 if you need to in order to support demand.