Yes to the first question. No to the second question. Yes to the third question. lol
What kind of filter are you running on the tank right now?
There's no such thing as being having too much filtration, but depending on what you're running right now, more filtration may not be necessary, even if you are overstocked. My 29gal will ultimately be a little overstocked and I'm going to be running a single Penguin 200B on it. The Penguin will provide enough filtration that even being slightly overstocked I'll be overfiltered.
Adding a new filter is simple...just set it up and turn it on. It will start accumulating beneficial bacteria right away while your existing filter will still maintain your cycle.