You should be able to do this: PC100 clocks down to PC66 just fine, but that doesn't matter -- the PowerBook G3 266MHz you have does, in fact, use PC66 memory (266 is roughly a multiple of 66 (4 * 66 = 264) -- if the memory was being used at 100MHz, the speed would be 250MHz or 200MHz or some integer or integer/2 multiple).
They both take SODIMM PC66 modules, so I would say go for it. If the RAM module clearly isn't the same size and shape as the socket, do not go any further. Use common sense and check the connections before trying to cram that memory in there. The worst that can happen is that the computer will refuse to boot/turn on and may complain a little.