Adding a piece of live rock will have very minimal effect as a filtration boost. You really do need the 1 to 1 1/2 Lbs of live rock per gallon to really get the benefit if biological filtration and denitrification. If you buy one small piece of live rock, dpdending on the quality of it, you will likely be adding Amphipods, Copepods, assorted marine worms, snails, coraline algae (encrusting algae that is pink, purple or other colors) and other possible surprises that could be great or not so good.
I'm putting a link here that doesn't sell anything, so I think it's not in violation of these message boards. If it's a problem, I apologize in advance and please edit it out.
This website page #1 links to page #2 and page#2 links to #3, all of which are very detailed in describing some of the most common, various good or bad creatures that could be on your Live rock.
http://www.reefs.org/hhfaq/pages/mai.../faq_rock1.htm
Do some research on cured live rock. You can buy fully cured live rock which will have minimal life on it and you can buy specific pods and other creatures you may want. It's a detailed topic. I will never have a saltwater tank without live rock.