Is it okay to install a second RAM chip? I have a 2010 MacBook Pro with a recently replaced logic board and SSD upgrade. I haven't yet upgraded the RAM. 

Currently, RAM is 4GB, one slot, Apple RAM (whichever manufacturer they use). I am thinking of buying some Crucial RAM to bump it to 8GB. Would it be better if I did 2x4GB from Crucial and put them both in the slots available, or could I get just one Crucial 4GB RAM chip and install it in the empty slot? Would there be any advantages/disadvantages to either?
Crucial RAM will almost certainly work with the existing. 

Their configurator is pretty good at getting exactly the right RAM for the machine.