Regardless of if you are a half-man team or a 200 man team you need to be clear about what your goals are. Which mean yes there must be project management.
Now you don't need to draft a two hundred page word document with signatures, and graphs, and links to further reading. In fact please don't do that.
But you do need to be clear about:
The challenge that you express is the following:
Faster alone, further together
- African proverb
First of all, you shall not see testing as a luxury for bigger team. Writing some unit tests along with the software helps you:
to avoid loosing time on finding bugs later;
to do more tests and thus deliver higher quality despite your few resources,
How to deliver fast and avoid growth challenges with a codebase?
These are commonly at odds with each other. Fast delivery can mean cutting corners and accruing technical debt, which leads to growth challenges when the codebase needs to adapt to new requirements.
The answer is a balance between the two, and it is contextual.