It is up to you, when you start with other units apart from fire units. It is easier and faster to generate a sizeable credit flow with fire units at the start, since the missions pay better, there are more of them and the progression is faster. Once you've established yourself (around 75.000 - 125.000 credit flow per day) you should be comfortable to get into other fields.
When should I start using units other than fire units?
Last Updated: 719d