Is mysql 5.6 fit for production use ? What is the difference between Release Candidate and GA?

Professional sysadmins need their code bases to be supportable, hence why we more than frown on people using prerelease code in production, in fact we wouldn't even count that as production inherently - it's the sign of an amateur.

So given 5.5.28 is the latest stable and supported release I'd stick to that rather than jump ahead into 5.6 territory. Oh and a release candidate means exactly that, that's a a candidate for release, very often it's identical to the released (and supported) version but sometimes there are late bugs fixed in fully 'Generally Available' (i.e. released and supportable) versions that existed in the RC versions.

Basically if you want to be considered professional don't be too keen to leap forward with code versions, you want the service to be available and your data safe, these objectives are usually easier to achieve with supported code as you can just contact the vendors and they'll support it and you.

The GA release would be considered stable enough to use in production, while the RC release is in the final stages of testing before general availability.

What this means for your application is:

Since the app you plan to develop needs features in the release candidate, it's fine to use it in development, provided you also switch to the 5.6 GA release as soon as it's available (and retest your app against it).