Stopping down for DOF certainly won't hurt when you're focused to portrait or closer distances. Parallax, there isn't really a good way around on anything other than an SLR. A rangefinder still has the viewfinder offset from the lens, so you'll still have to correct. In general, you can do this by framing the same number of inches off in the same direction as the offset -- so, for instance, with my Moskva 5 I'd frame about 2 inches right and an inch and a half high, to compensate for the lens axis being that far below and left from the viewfinder. It's not really a big deal unless you're trying for very precise framing or working very close.
When I was doing this stuff with my Pony 135 I completely ignored the viewfinder, because at the close distances I was focused the flower that was the subject was barely in the window at all.
A few 35 mm rangefinders, from the 1960s and later, have parallax compensating frame lines in the viewfinder, but those are only right when you're using the unmodified lens with the rangefinder. You could try mounting a parallax compensating viewfinder attachment on an accessory shoe, but then the direction and amount of offset are set for the brand/model of camera the VF was made for -- which was likely a compact 35 mm like a Leica or Contax.