It's never recommended to mix RAM - even of the identical model as it is NOT guaranteed to work without issue. The only means to be sure that multiple DIMMs will function without problems is to buy ONE matched, tested RAM Kit. With the current low prices on RAM I would suggest you buy ONE matched, tested RAM kit of the full quantity you desire and NOT mix in any of the RAM you have. While some folks have successfully mixed RAM many have found it impossible and a waste of money.
Always pick RAM that is approved for the specific brand CPU you are using as not all RAM works with all CPUs. 8 GB. of RAM is the ideal for most people. A 2x 4GB. kit is around $60. on Newegg depending on speed and model.