Too much evil or good will cause one or the other to leave with no chance of persuasion. This recommended combo will also ensure that you can keep the female starting Settler if you pass a very hard Speech check (that can actually fail with 10 Charisma!), and an easy check can let you keep Barstow operating her Overseer's Desk for bonus Happiness. I'd personally recommend the good Power Cycle, the neutral Soda Machine, the good Phoropter, and the neutral Slot Machine, although the evil and good variants of the gambler are safe and have significant benefits too.
Oh! And be sure to check Overseer Barstow's terminal after each quest, as she makes new notes of her own on the test results. The material costs and resources generated also vary between each version, so there's actually quite a bit to consider. The experiments that can randomly kill your Settlers who use them are the evil & neutral versions of the Power Cycle, the evil version of the Soda Machine, and both the evil and neutral versions of the Phoropter. Do note that the experimental R&D notes in the terminal about fatalities are NOT just flavor text, but actually telling you about the danger these devices can pose! There's basically evil, neutral and good choices for each one (in that order), and advantages and disadvantages to each. How useful or dangerous the experiments are after the quests is determined by which parameters you had Clem operate them under.