My 13 weeks old Havanese growls and cowers when she meets new dogs, big or small. She is only just vaccinated enough to be taken to parks etc and I am starting puppy classes with her this week so hopefully that extra socialization will help. My question is, how should I react to her when she does this? It does appear to be a fear growls more than aggression. Do I say "no" or do I try to reassure her somehow? I don't want to reinforce the behaviour but also don't want to reprimand her for feeling scared. Also, is that a fairly common Havanese trait, or something more specific to my little girl?
It's not a Havanese "trait" for sure. You ARE starting her socialization on the late side, though certainly better late than never. And I am quite sure you are right, it is fear, not aggression. If she wear my dog, I'd try Very hard to find some friends with older, well socialized,non-rambunctious, puppy-tolerant dogs and just let her spend time with them, on the ground (or floor) not in your arms. SHe may growl at them to start with, but if she has room to get away from them, and is left to figure it out, she will start to see that they are no threat.
If this isn't possible, take her,on leash, to somewhere that she can observe other dogs from whatever her "safe distance" is. Don't make her get so close that she feels the need to growl or try to get away. give her LOTS of cookies and praise her HIGHLY. Over a LONG period of time, as she begins to tolerate them better, move a bit closer, but still within her comfort zone, and repeat.
This is ALL I'd do with her without a professional to help you. Your puppy class, if it is well-run, will help you too.