The answer regarding when or if to talk to police is a simple one to answer. If you're only being called as a witness then by all means proceed with a statement relative to what you might have witnessed.
If they want to question you about something and it doesn't fall under the first heading then be polite and decline answering until you've obtained representation. Why? Because your attorney will protect your rights, inquire and in short order ascertain their degree of interest and your potential vulnerability and because if you're a suspect you aren't going to convince police of your innocence by cooperating.
So there's little upside to cooperating. The downside? You're going to be nervous. Even if you're innocent. It's human nature. You can be riding down the road at the speed limit and a patrol car hits the lights, what's your reaction? Until he passes you the same one you'd have if you'd been speeding. Those nerves can lead you to say stupid things, to mistakes in recollection or tiny lies to bolster the innocence you know is yours and all of that, all of it can be used against you later to impeach your credibility. It could be the difference in a perception of guilt or acquittal if things go really wrong.
I know any number of police officers. One of my favorite human beings just retired from the pd in California after a full career as a military MP and he'd tell you to keep your mouth shut too.