They will take over your house! To keep a line going for a long period of time and keeping the power to maintain multiple lines so they don't get inbred which results in smaller fish and deformities after a while... The minimum most sites suggest is 5... I just keep mine for display, don't have room to keep the quality up.
They don't have the round tail... Lots of amazing fish, make sure you check with someone though, I've never asked her, I've just been told by someone before, I may be COMPLETLY wrong.
http://www.goldpisces.com/premium/guppy-galaxy.html