Generally not a good idea to mix RAM vendors. But sometimes necessity if the mother of invention. As some older systems pass into obsolescence then finding matched or vendor approved RAM might proof either very expensive or impossible. Having at least one known stick that works would allow you to try out other brands. The simple equation being that if they are stable together then the vendor ID and chip maker is not an issue.
I would download Memtest86 and run it to be (close to) 100% sure -if it passes then you can rest easy. A quick cut and paste from one of my earlier threads....
-Download Memtest86+ here
http://www.memtest.org/ then burn the .iso (after uncompressing) to CD with ImgBurn or burning software (needs to be bootable) OR download the executable for USB if your BIOS allows you to boot the system from USB
-If you have the RAM set at non-stock levels it would pay to set your BIOS to default settings.
-Power off. Remove your 3 known working RAM sticks and insert the new stick into DIMM 0 (the first DIMM slot)
-Reboot computer, change boot order to CD (or USB), save change, reboot.
-Memtest will load and test the module. Each pass takes around 10-15min to complete for 1Gb, run 7-10 passes minimum. Tests 2 and 5 within the pass are most likely to find errors and you can order loops of these tests in the options. If the RAM passes, stop the test and power off. If the RAM fails then stop the test and skip the next step
-Move the RAM stick to the 4th DIMM, reboot and repeat the test sequence.
-Stop the test when you're happy that the RAM is stable, power off.
-Insert your stable RAM back into it's slots
-Remove CD/USB, reset boot order, save changes, reboot.