It is flexible but the word order controls the meaning, the focus of the sentence.
That is much more natural with negative nikde rather than with the positive někde.
Nečekají na něj nikde. - The are not waiting for him ANYWHERE.
Čekají na něj někde. They are waiting for him SOMEWHERE. (??)
I had a weird thing happen with the listening version of this sentence (type what you hear).
I mistakenly typed "Někde na ho čekají", and got the message "You have typed in English, not Czech".
This happened twice, before I typed the correct sentence, which worked as expected.