Questions & Answers about Die Bullen sind schon da.
Bullen literally means 'bulls', right? How strong or offensive is this slang for police?
Yes, der Bulle means 'the bull'. As slang for police, it translates well to 'cops', but it leans a bit more derogatory, sometimes closer to 'pigs'. It is heavily used in colloquial speech and German crime shows, but it is considered disrespectful. Never use it when actually speaking to a police officer! The polite terms are die Polizei (the police) or die Polizisten (the police officers).
Is Bullen plural here, or is it one of those nouns that gets an '-n' in the singular?
It is plural here (notice the plural verb sind). However, you are right to suspect a trick! Der Bulle is a weak noun (often called n-declension). This means its singular accusative, dative, and genitive forms also take an '-n' and become Bullen (e.g., Ich sehe den Bullen - I see the cop).
Why does the sentence use da instead of hier for 'here'?
While hier means strictly 'in this specific location', da is much more common in everyday German to express that someone has arrived, is present, or is available. Saying Sie sind schon da is the most natural way to express 'They are already here' or 'They have arrived'.
If I replace the slang with the standard word Polizei, does the rest of the sentence stay the same?