My opinion: That is fine. The only thing that you're probably expected to do is committing to you own proposal. This means that you should at least ask or answer 10 questions in the private beta of your proposal. But if you after that only use the site for some questions or answers now and then, it is not a bad thing. Furthermore, you should create five example questions (well, at least three to get started) for your proposal in the definition phase to draw attention to it.
Some data from other sites:
- Mathematics: proposer has 2 posts and has 379 rep.
- Statistical analysis: proposer has 312 posts and 21.8k rep.
- Theoretical Computer Science: proposer has 9 posts and 663 rep.
- Physics: proposer has 64 posts and 2.7k rep.
- Chemistry: proposer has 0 posts and 101 rep.
- Biology: proposer has 12 posts and 655 rep.
Two thirds fullfiled commitment and only one third remained reasonably active on the site. So it isn't very rare to have just a low activity on the site.