Please don't take this personal, but posts like this read "like a lamb walking into a den of wolves" ;)
I believe this type of forum would best help you on determining what type of usability features to think of, how to engage with a design firm, or which attributes to look for in an interaction firm. There are many good posts on these topics in this forum - take a look through the tags.
When it comes down to it, the type of firm you select is closely typed to your target market and your specific views about your product - so while I may have had a great experience working with a firm on streamlining conversions in an ecommerce project, that doesn't mean that they will be successful with your tween oriented crowdsourcing site (if that's your target).
Good luck, and happy reading!