Most RTS start off pretty passive, but they get aggressive as they grow larger and older. Sorta like people getting crotchety in their old age.

Mine was in my peacefull community tank for quite some time until she started growing bigger than the other fish. That's when she changed, almost overnight. Extremely territorial now, even in my semi-aggressive tank.
As far as liveaquaria, I haven't personally ordered from them but know others who have and have been very happy with their purchases. They are a subsidiary of Drs. Foster & Smith, who I get a majority of my fish and dog supplies from. The only time I had a problem was when FedEx lost a package and Drs. F&S immediately shipped me a replacement at no charge and took care of tracing the lost package themselves.