He means is to get a speaker from the same family.
That way the sound and timber from the speakers all match.
This makes the front stage seamless.
You can look on the Polk TSi page for a list of these speakers.
I did not see the CS10 or CS20 on the site you provided but either one of them is what you would need to match your system.
You definitely need a sub that will get down to 20Hz. Lower is even better.
That's not going to happen in this budget range.
Since you are looking to put together an HT setup, look for 12" subwoofers because they have more output. Popular choices here at the BIC F12 and the Polk PSW505 because of the price/performance value. But your prices will be different, and there will likely be other better for you. See any 12" subs that look interesting to you?
The Rythmic subwoofers are the best by far for their price IMO. They sell only online. Shipping might be very expensive. The FV12 sells for $549 here.
Other excellent ones I know of are the NHT B12D subwoofer and the PSB Sub Series 300 and the Monitor Audio Bronze BXW-10.
by the way i have to pay 33% from the product price to have it its a tax other than shipping