I'm thinking of using SO as a support forum for my open source projects. Is that allowed?
Generally speaking, no. See:
Use Stack Overflow as the official support site of an open-source project
Is it okay to use Stack Overflow as the support forum for a product or project?
is it possible to create tags before questions have been answered?
Can I create a question/answer for a question that I usually get to create a tag?
As long as the question is appropriate for stackoverflow, go for it! If you add enough good questions then people will naturally come here for programming questions regarding your project due to google.
However, as the above discussions point out, Stack Overflow cannot truly be your main support site. Programmers might be able to come here to resolve programming problems they face with your project, but you'll need to do development, bug, and similar discussions elsewhere.