Voltage, Latency and timings are not important factors.
... Well, Voltage is very important - but if you get standard grade desktop commercial components, they should all meet the standard. It is just some overclocked or special server memory uses different voltage, however they are usually more expensive and warn you.
Latency and timings, there are rare instances where it is not possible to have differences - usually some chipsets/motherboards prevent this, however, in most instances you will just get a warning when you start the machine and it will operate at a low speed (or even a penalty speed where it is slightly lower than the lowest - only seen this once on a American Megatrends BIOS years ago).
As it is part of your question - it shouldn't cause any damage to the machine. The machine is designed for the memory socket. At worst you will get a beep warning and/or the memory will run slow. I cannot imagine any irreversible damage. Even if you had memory requesting higher voltage, I could imagine the memory stick failing before the motherboard - but don't test it or quote me on that!
As for buying, my guide will always be to try and buy a matching / identical second stick as this enables dual (or higher) channel memory support, if your motherboard supports it. This mode can (under some circumstances) dramatically increase memory performance.