Can I mix brands of chainsaw chains?
I'll defer to the experts but personally, I wouldn't mix brands of chainsaw chains. 

I seem to recall that Stihl chain designs help reduce/prevent kick-back and binding (which presumably explains why they're more expensive). Plus, they use a green and yellow color-coding system to help you select a powerhead, bar and chain combination that complies with kickback requirements of various safety standards (e.g. ANSI Standard B175.1). 

But again, I'm no expert -- just paranoid about safety. =)

-M