I would like to know if it is possible to have some random answers, but also an answer "Others" and "Don't know" appearing these two at the end (so these two opcions wouldn't have that random order).
LimeSurvey does not support these typical requirements out of the box.
(Feature request from 2009 is here :
bugs.limesurvey.org/view.php?id=7367
)
At least if you don't you the automatic N/A, which enforced N/A on every question with required answer.
The typical solution is a workaround customized to each question(type).
You can find solutions in the forum and in the manual. Check if the workaround is suitable for your LimeSurvey version.