How should a shul determine its hashkafa? Is it the hashkafa of the community at large, or the rabbinate - even if they do not always seem to be identical?
What you're defining would be a cult, a group of people in a community who all believe the same things, etc. In a healthy religious community, and I'm not even just talking about a Jewish one, you will find people of differing opinions who come together to worship and serve God. There may be certain boundaries within a community for what is accepted and what is not, but you will find people on different levels and on different points of a spectrum. (And if you don't, I recommend getting out of there.) To answer your question more precisely, Jewish individuals within a community should base their beliefs on Torah and mitzvos, and what is true and right.