6500 PLN Gross is enough to live and keep spare for a small family parents and a small kid ?
You could live on that but it wouldn't be a particularly luxurious life. You'd need to allow at least 2,000zl per month for a three-room apartment in Wroclaw, and that would be close to half your net income.
how smooth is family reunification to be processed for a person that has work permit and residence card.
Depends what family you are talking about; if it's anybody other than spouse or kids, you can't really import them.