Having a slang word for a Pole which origins from the original description of a Pole isn't really that offensive
Well, that depends, really.
For example, a slang word for a Russian in Polish is "Rusek/Ruski". Adjective for anything Russian is "ruski". It of course originates from Russian language in which "ruskij" means "a Russian" or adjective "Russian".
But most Russians usually find it offensive. And often they are right to do so. It sounds even worse if it's about a woman ("Ruska"). Of course, many people in Poland probably got used to this word to such an extent that they don't realise it can be viewed as derogatory. Some russophiles also can use this word but in a positive way, to show affection.
I guess it depends on intenstions.
Another example - a derogatory term for a Chinese man in Polish is "Kitajec". "Kitajec" is a normal, official Russian word for a Chinese :)
So, you have to be careful with words :)
In general, I think, those Poles in Poland who know that Polish people in the US are called Pollacks, wouldn't want to be called this way.
As for German words "Polacke" and "Polacken" (at least in case of a German saying them) I would advise to use them only among Polish friends if this person knows they won't feel offended.
What is the correct word to call Polish people...????
As far as I know in English language it's "a Pole" (singular) and "Poles/Polish people" (plural) :)