It's best to keep them at the same speed - results may be unpredictable; subtle, rather than system lockups, failures, etc.
You may find that there are only occasional glitches - the best case scenario is that the new PC100 memory simply runs at 66Mhz and puts up with it.
You can be a little more certain by using quality memory (Kingston, Crucial, etc).
If it's just for one machine, my suggestion is "suck it and see". If it was for a major corporate roll-out, I'd say forget it.
Hope this helps