No, don't worry about that.
if for example your old samsung ram ran at a different speed than your new stuff, the new RAM would just be slowed down to the speed your old ones are (2700) because of the way Ram technology works, as it couldn't force the old stuff to run at a faster speed, and its just the way the Bus (what carries data to places) works.
Sorry if I confused you though, as this shouldn't be a problem. You don't have to change speeds.
Just try with each stick of ram until you find a stability problem, and then you know which one caused it. You can then move onto finding out if this problem occurs when you have the old ones in, with that one stick of Elixer Ram that is not causing a problem.
As I think it definetly a problem with one of them and might have been bad luck that you tried the same one on its own that might have the problem.
You could also try differnt ram slots if this doesn't help you
I hope this helps