Always hire brainers at the start of the month, so that you get the most value out of their salary. Don't hire them mid-month unless you are rich. (You have to pay the full salary even if you hire them on the 30th and they haven't arrived yet.)
With that, brainers aren't as big of a cost as they seem. Their research generates a large amount of points, which translates into a large cash bonus at the end of the month. At the same time, you can use the research to generate new money making opportunities. ie. Warehouse missions, animal hunts, undersea missions, bounty hunts, etc. are all early-game techs which will give you more money, and can get them sooner if you have more brainers...
In previous playthroughs I've experienced pretty heavy financial stress from hiring too many brainers too soon; but in my current playthrough I've found that I can just hire as many as I can buy at the start of the month; because the end-of-month bonus pays the bulk of their salary. (That said, there's a bit of luck involved in that currently, due to the negative score bug thing. At least, that's what I've been told. I haven't noticed it happen myself.)
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By the way, there are ways to micromanage your research to avoid wasting brainer-days, which is pretty important in the early game. But I'm starting to feel like everything I've posted so far is a spoiler, so I'm just going to leave it at that.