Although mixing isn't likely to create any problems, you are aware that the entire system will slow down to accomodate the slower RAM, right? Anyone else reading this should be aware that a couple of years after the last P3 was made, there were changes to PC133 RAM SIMMs such that those were no longer backward compatible with older PC's intended for PC100. I wasn't keeping up closely with the hardware situation at the time, and the details are pretty vague in my (aging) memory now . .
🙁