You didn't indicate your location, but I assume you are in south FL or southern CA as Ficus will freeze any further north. They won't get very large here in Orlando, but in south FL they get HUGE. And yes, their roolts CAN be very invasive. I'd recommend planting it at least 15' away from any concrete, such as driveways, houses or sidewalks. Also keep in mind where your sewer line or septic system is located and avoid that area.
I need to head down to Ft. Lauderdale this month to trim a Ficus for a friend, it towers over 2 houses and hangs over the road and the yard across the street!