You don't need a project manager to apply formal project management, and it's doesn't create that much of an overhead.
Even if you're a 5 person company, you'll soon discover that applying ad-hoc project management will quickly get out of hand, and you will feel that you really need some formal project management.
Mini project management does not officially exist, however, if it does, I see it as applying project management with the minimal overhead possible:
- Concise project documents (including the project charter, the project plan, and the lessons learned report)
- Short week meetings
- Simplified reporting
Ideally, to do all the above, you should use a project management tool (there are some free, but I'd go with one that is paid, they are not that expensive, and they are more reliable).
I would stay away from Excel to manage any project, even if it's a very small project in a very small company. It can get really messy once you start making changes in your project schedule, and you won't be able to keep track and compare your current schedule, costs, and scope to your original baseline.