Ram Memory upgrading 8 to 16 can be different brands>

In general, yes you can mix brands, but sometimes it doesn't work even if you match other aspects of the memory modules (such as speed, CL timings, etc.), and unfortunately there isn't always a way to predict whether a specific combination will work.  However, one thing you can try doing is using Crucial.com's Memory Selector Tool, since it will allow you to specify your laptop model and then display modules that are expected to work.  Of course this will only show Crucial brand memory, but in my experience, I've found that this Crucial's recommendations are very reliable, so it should improve the likelihood that the new module you buy will work.