Some DDR memory does not like running at slow speeds.
However, your biggest issue is that on most motherboards you CANNOT mix registered and unregistered DIMMs.
All memory modules either need to be registered or unregistered.
It is "best practice" to have all modules of the same speed and CAS latency.
Perhaps the most important factor for having multiple modules work well together is that they be of the same chip density.
That is, all 512MB modules should have the same number of memory ICs on them.
256MB modules of the same density may have 1/2 the number of chips.
1GB modules would have twice the number of chips.
Still, bottom line, your 2 DIMMs will not function together on all but a few motherboards.
Chas