You can have those memories running together with pretty high confidence, but at what timings, is completely another question.
As a rule of thumb, all the memories must be run at the timings of the slower memory. So in your case 5-5-5-15. You didn't specify the T timing of your 512's, but the slower one is again the one that matters.
Also, adding DIMMs to the bus may have adverse affects. Depending on the quality of your DIMMs (both new and old), you may be able to run with tighter timings, but if you're out of luck, you may need to drop the timings even below the slower one's timings to get the computer stable. Especially your ability to OC (if you've done that), will most likely drop.
Also, the make and model of memories matters. Avoid mixing DIMMs from different manufacturers, and even different product lines. If you have high-performance memory, don't buy value RAM.
hth,
lehter