In a QnA site for religion only answers with references to credible sources should be allowed in this site otherwise they'll be chaos.
There should be no personal opinion questions or answers.
The biggest problem with the "credible sources" issue is that not everyone agrees as to what constitutes a credible source. Although the Qur'an itself would be considered credible by pretty much anyone calling themselves Muslim, it is written in Arabic whereas this site will cater to a predominantly English audience. There are many English translations available and they are not as unanimously accepted, nor do they always agree with each other.
Many hadiths which are deemed credible by Sunnis can be outright rejected by Shi'ites, and vice-versa. And that's not even getting into the Qur'an-only movements which don't accept written hadiths at all.
Would amateur translations of otherwise untranslated texts be credible?
What about scholarly opinions? Shall we only accept the classical texts, or would learned opinions of modern-day scholars also be acceptable? Or would any scholarly texts written after the death of the prophet be deemed non-credible since they weren't part of his sunnah at all? Sound arguments can and have been made for all these cases.
The fact is, there is such ambiguity as to what is or is not a "credible source" that any attempt to programmatically enforce it is non-trivial; trying to do so will likely be far less effective than just using the community to ensure quality, as is done with the other StackExchange sites.
The site should be for experts in Islam. As with any other Stackexchange site the community must ensure quality. This can be done by adding references to credible source in answers (or questions for that matter) or by flagging answer with poor quality (speculative/subjective answers).
I think it is worth to discuss what a credible source is. I do not believe that it makes sense to restrict references to a fixed set of credible source. I think this should be decided by the community through high quality (and upvoted) answers.
I think personal opinions can add much to an answer (this often occurs for instance on http://programmers.stackexchange.com) as long as it is stated clearly what is personal opinion in the answer.
This said, I do think that with any site on religion a major task of the community is to ensure that it does not turn into a discussion board, but remains a Q&A site.