Eu resolvi assim:
KERNELS=="1-2.1", SUBSYSTEMS=="usb", ATTRS{idProduct}=="6001", ATTRS{idVendor}=="0403", SYMLINK+="usbS0"
KERNELS=="1-2.2", SUBSYSTEMS=="usb", ATTRS{idProduct}=="6001", ATTRS{idVendor}=="0403", SYMLINK+="usbS1"
KERNELS=="1-2.7", SUBSYSTEMS=="usb", ATTRS{idProduct}=="6001", ATTRS{idVendor}=="0403", SYMLINK+="usbS3"
As vírgulas e as diretivas SYMLINK fizeram com que funcionasse.