[gcost]

Do startups build their own tools at all? At what stage do they usually start building custom tools?

[wmf]

Almost all good tools were built for in-house use first. Rails is a classic example and there's a front-page article today about the history of Cassandra. But you should only build a tool if that's less work than learning/adapting an existing one, which is almost never. Either existing stuff has to be really bad/nonexistent or youhave to have an idea(+execution) for how to make something 10x better and a need for 10x better.