DISQUS

julien, it sounds like you are a bad need of a partner ;)

it's true that areas where you are weak are better left to experts... so until you find the "one" who can complement your skills, it is a valid idea to use existing software and it being open source would leave you some flexibility, so that's a good compromise in the end :)

very interesting to follow your implementation process.

i would lean more towards creating and maintaining your own code, especially if the piece of software is a central part of your software, in case something happens it makes troubleshooting much more efficient.
however, i think that if you are using code from a well maintained source, then that should be safe, i am especially thinking of cpan for perl as an example but not sure if there's something similar for RoR. Using published APIs from the company's website should be safe too i think.

but to me, i go with the motto that "you are better served by yourself" so you are better off doing it yourself! Especially for a startup, I think that if you can pull it off on your own, you'll be even more proud of your product at the end. Good Luck!