From an evolutionary standpoint: traits that increase survivability are the ones that survive, right?
In a fight, freeze, or flight response, you make it sound that this race values the flight response the highest. Meaning that social pressures would reduce traits that increase the fight response, so stronger, bigger, sharp teeth, will probably not be passed along.
The freeze response will be taken into consideration, because possums are a decent real world example of it working. IF the race is easily frightened, this will probably be the response that is most commonly used, meaning that adaptation traits would follow this.
In a race that tends to freeze in the face of a threat, the traits that will allow survivability will be a couple of the basics.
if the creature faints and then skunks its predator, it will increase its chances of survival.
if the creature faints and tastes bad, it's not getting eaten, also increasing its survivability.
When freezing up, if the predator senses prey by movement, heartbeat, heat, or whatever, if the prey can diminish its detectability nearly instantaneously, it will survive.
The flight response would also be used by a pacifist creature. If they had strong wills and had any advantage over their predators, this response be the one that would probably increase the chances of surviving. Some of the traits that would follow this response are as follows.
The old adage "I don't have to be faster than a bear, I just have to be faster than the slowest person" would come into play heavily. Faster ones will survive, slower ones will die. The race will slowly become faster and faster.
If they can swim better, if they can fly, if they can tunnel faster, swing from the trees, or jump longer distances, they will survive. The ones who use their environment better, along with their biological advantage, those are the ones who will survive.
No matter if the creature faints or flees. If it is blending into its environment, guess what, that's increasing its chances of surviving.
I know this would be primarily thought of as a fight strength, as bigger size means that it would be able to fight off predators easier. However, in a freeze response, the smallest ones would probably be the ones to survive. If I am chasing two dove for example, and they both faint, I'm taking the bigger one, not the smaller one.
The ability to sense danger increases the race's surviving. Whether it is hearing, smelling, or even sensing, advantages in one of the senses will enhance the race.
Size can come into play in a flight response as well. It kind of depends on the situation these two races belong to. If we look at the human/mammoths, the bigger ones survived due to the ability to withstand more attacks. If we look at cat/mouse, the smaller ones survived, being able to hide in more places.
No matter how you react to intense situations, how intelligent you are will change your chances of surviving. Curiosity killed the cat.
Obviously, the race's society will play into their surviving as well. A couple of things will allow them to survive, BUT as their societal surviving ability increases a few negatives come in. Look at humans, our society is what allows us to survive, put one of us with minimal gear anywhere, and we're pretty much dead. This will happen to any other race that relies on it's society instead of biology. This isn't necessarily a bad thing, it does mean that if the society is willing to cut out the weak, or those who go against it, those members will die. Along with this, as the society develops larger and larger, it's death rates as a community will increase substantially. A 1000 person community might lose 1-5 people a year for example, a 100,000 person community will probably lose 1000-2000 people a year. Here's what societal traits will increase the races survivability.
If they are split up into tribes of 50-100, as opposed to one group of 1000, they will die easier. Strength comes in numbers.
As a community, how fast they develop ways to increase surviving will obviously change their survivability.
The biggest reason ants have survived as long as they have is because they live underground. Similarly, a race that exists on a different plane (ground, sub-terrestrial, super-terrestrial) as their predator has a better chance of surviving.
The ability to communicate with the predator in any way will increase survivability. This might be hard to understand, but tricking your prey is increasingly easier as you can be understood by them. This plays into intelligence as well, but can be considered its own trait.
You have a lot of thinking to do about the dynamics of your two races, here is a small list, if you have a more precise example, feel free to come back and I will help you understand the dynamics a little better. Hopefully this list helps, but there are many more traits that can increase or decrease survivability. When in doubt, look at nature.