It's not usually a good idea to mix brands if at all possible. That said, I do it quite often with no problems. If they have similar or same timings you should not see any performance issues. If they weren't then the computer would default to the slowest speed.
Generally as mentioned it's not a good idea to mix-match IC's. This can be an especially difficult problem if mixing Frequencies and Voltage -- neither are recommended. The capacity must be the same per channel e.g. CPU: | Ch0 | Ch0 | Ch1 | Ch1 |
CPU: | 1GB | 2GB | 1GB | 2GB | Ch0 = 3GB & Ch1 = 3GB so Dual Channel and all memory is available. Conversely if it's not then the amount of RAM on older MOBO is the lowest channel * 2.
RE: "Is there another name for CL 5-6-6-18" -- CAS 5 RAM with oddball timings, often seen on EOL OCZ kits and (PC2 6400) DDR2-800 frequency.
IF mixing RAM then install each set in it's own color DIMM slot and if it were me only on a non-OEM MOBO where you can manually tweak the Frequency, CAS Timings and Voltage.
Assuming this is from a DDR2-800 kit follow this example:
DDR2-800 5-6-6-16 @ 2.00v
DDR2-800 5-5-5-18 @ 1.80v
DDR2-800 5-6-6-18 @ 2.00v ; use the worst of each
/edit - another important note: Review the MOBO or OEM's memory and supported OSes specifications before adding or replacing RAM. Especially note on some older MOBO chipsets 4GB is the limit regardless of OS. Also to access >4GB requires a 64-bit OS and I really don't recommend Windows XP 64-bit.