You are talking about two subjects here. On DOF - speaking in general terms, you will get a shorter depth of field with a long lens, compared to a lens with a shorter focal length. That's why long lenses make such good portrait lenses, you can isolate you subject from the background with a short DOF.
The rule about shutter speed being no lower than the focal length of your lens is generally an SLR rule. SLR cameras have a mirror that slaps up when you trip the sutter, which can make it tough to hold the camera steady - especally for long exposures. RF camers have a different type of shutter which is much quieter and has less mass - which means less camera shake when fired. Because of this you can easily hand-hold an RF camera at 1/30th of a sec - even 1/15th if you steady yourself on a solid object. Of course, if you're using a 90mm lens on your RF, you may want to stick to 1/60th or faster.
