He can certainly be sued. Anyone can be sued by anybody else.
Au contraire. The SYG law in Florida provides for IMMUNITY from arrest, prosecution or CIVIL LIABILITY. If self-defense grounds are established - dismissal of all charges is ordered.
By extension, the Homeowner's Association arguably stands in Zimmerman's shoes in that if his actions were legal then they're equally immune from liability. ;-)
There's a big difference between being sued and being held liable after all is said and done. There's nothing under the law -- including the "Stand Your Ground" and "Castle Doctrine" principles -- that prevents Martin's family from going down to the county courthouse tomorrow and filing a civil lawsuit against Zimmerman.
How the case ends up up being adjudicated is a whole different matter, and I'll venture to guess that it would be years before all of the motions and appeals work their way through the system. It's also unlikely that any civil trial would proceed (or even a lawsuit filed in the first place) until the criminal proceedings are done.