If ya have a sponge filter that's cycled on your 17g already just do bi-weekly water changes and you'll be fine with just that filter. No need to over filter the tank. Just don't over clean the sponge filter and you'll be fine. Only clean it when need be, if it's still pumping out multiple bubbles per second it's still good. When ya clean the sponge just make sure to use used tank water not tap.
Yes. If it's a new tank you always have to cycle it. Takes roughly 6 weeks to cycle with no fish in it.
A fish in cycle still takes roughly a month. With a fish in cycle it's best to test the parameters of the tank daily and give it a 20% water change every 3-4 days. With a small tank like your 17g I would recommend a 10% water change every 1-2 days along with a 20% water change at the end of every week.
