Small to small is known as cross-chaining, where the chain is as bent as it can be given the bike it's on. The other cross-chaining combo is large to large. And yes - these may rub the FD. But you shouldn't be riding in small/small or large/large either. It's bad for the whole drivetrain.
Shimano nowadays includes a chart with their FD's to show you what combinations are most likely to rub. I enclose a copy below:
