[nilkn]

Unit tests and basic code reviews aren't exactly exclusive to enterprise-level system architecture these days.

Regarding unit tests, their utility is actually mostly independent of the size of the team. The more relevant factor is the size of the codebase. A small team can end up producing a pretty huge codebase, and solid unit tests can end up saving a lot of frustration in the future. They also can be critical in helping new developers familiarize themselves with the codebase and its interdependencies.

Code reviews are an investment not just in the code and the product but also in the human capital producing it. One thing you'll learn with experience is that even very good developers will write bad code sometimes. If you've got millions of users, simply doing code reviews can be a lot less stressful than finding small mistakes later on when bugs pop up in production and a hotfix has to be pushed. It leads to less blame, fewer production bugs, and a more collaborative, academic environment. People can learn and grow a lot from code reviews (both receiving and giving). They'll improve the product and codebase not just in the short term, but doubly so in the long run.