Is it somehow possible to adjust how much health can citizen have?
Through save-file editing (save-files are plain text, just use Notepad), you can give your colonists a maximum of 150 health. All you need to do is replace one of their traits with this one:
<Trait Class="Trait">
<DefName>Tough</DefName>
</Trait>
It may be possible through editing of the Assembly-CSharp.dll file, but I do not know as I've never touched that file.