I use an RZ for street photography, mostly with the 110/2.8. Your 90 or a 127 or whatever will be fine. Of course you aren't going to be taking many shots in secret but that's OK, you can still get good candid photos. I always use the WLF, some fast film (TMY2 at 800 in XTOL is good) and mostly shoot wide open.
A tripod is a good thing to have - even if the camera is not resting on it, having the legs dangle below the camera provides a lot of damping of the mirror slap so you can comfortably shoot handheld at 1/125 and get sharp results. And of course you can shoot on a tripod quite sneakily - setup a shot focused on a particular spot (e.g. if you want to juxtapose a particular demographic with a particular sign), put the cable release on, sit off to one side and wait. Once someone comes into your frame you can release it without peering into the camera and making it obvious you're about to take a photo - people think the camera is just sitting there doing nothing until it's too late.
Some examples (thumbnails are links):
Edit: first two are handheld shots, TMY2@800, XTOL 1+1, subjects quite aware of my presence, very dim late evening light (1/125 f/2.8). Third one was on a tripod, waiting. Fourth one was at night, shot handheld on Delta-3200 at about 1/60, EI1600.