If I have an idea for a site, but want feedback from other SE users before building the community and officially submitting a proposal on Area 51, is it okay to ask here?
Yes, you can. You can use the community-planning tag to indicate that you're asking for help in crafting the proposal before you make it.
That said, you should be as clear as you can with the question; a question of "Should I propose a site for [X]?" with no real details probably wouldn't be a very useful question, and probably wouldn't get many good answers.