Can I use (personal) pronouns in imperative sentences?
eg.: Ne lasu lin iri! - Don't let him go!
Could I write these? - Ne vi lasu lin iri! / Ne ni lasu lin iri!/ ...etc....
Esperanto is about communication. If it makes it more clear to put in the pronoun, then do so. If is already clear, you can leave it out. Think of it in terms of clarity and emphasis.
I wuold say "vi ne...", "Ni ne..." so that "ne" negates the verb and not the pronoun --- but yes.
The meaning would be something like "you shouldn't let him go" and "let's not let him go."
And if I want to say, that: "We have to stop him", (or Have we stop him) (?) then I Could say?: "Ni Provu lin ĉesi"
Must I use it the "ni" if i want to say that: "WE" need to stop him"? I don't know, how is it working in English too. Can one use the "we" with imperative sentences in English?
"ni provu lin ĉesi" 'let us try to stop him' is correct but sounds a little weak ; I'd try "ni ĉesu lin" or possibly "ni devas lin ĉesi" ('we must stop him', which is not imperative)
In English the simple imperative, where you front the verb and drop the subject, "Stop him", really only works in the second person; we usually have to translate it with forms of 'let' or 'should', which aren't as strong and are interfered with by the ther meanings of those words
"ĉesi" isn't transitive, so you can't say "ĉesu lin" or "ni devas lin ĉesi".