I have Shimano WH RS30 rims which are 700c x 32. I would like to use a beefy 35c tire. Is that too much width for my rims?
(My forks and brakes are wide enough to handle a 42c)
I've seen and used tires with more than twice the width of the rim without issues. Using a 35c tire on a 32mm wide rim should be no problem.
Besides that, it is interesting to note that here
it says these rims are 522x15c, and the rim width is 28mm... Are these the same model as yours?
Assuming that your rims are actually 622x15 ISO size, like the Shimano page for those rims says...
According to this handy chart on Sheldon Brown's site, putting a 35mm tire on a 15mm rim is slightly bigger than recommended. It's close enough that it should be fine, however. That chart is fairly conservative (and even says so at the bottom).
As far as the widest, you could put 42mm on there if the fork, brakes, stays, bridges and seat tube all have the clearance, but that's really pushing it quite far and likely to cause poor handling or other issues. 35mm is just barely pushing it and should be fine.