I hope this no longer an issue for you however, as I understand it having unequal Ram size is not a huge problem, but you must make sure the Manufacturers are compatible, and operate at the same speed, or you will end up with instabilities with the system.
Such as, "Crashes, Freezes, and eventually over longer periods may even do damage to the MotherBoard, where over-heating can possibly occur. The reason this can happen is because one slot is working overtime and there is often a tendency for the machine to run much slower than it is actually capable of doing...
This is just my understanding having used Macs since 2002, and having replaced most of the Ram, Hard drives and Batteries myself.
I am by no means an expert in the matter, however I have had to learn the hard way on more than 1 occasion, which I can assure you tends to make you think twice about doing things without a little research into the possible outcomes 1st, that said, "if it aint broke, don't bother to fix it... Hope this has helped in some small way, Best of Luck Pal.