I can tell you how I determine which breed is the ideal, but I am not sure if there is a true set in stone rule for a "correct" breed, even though we sometimes agree that certain breeds work better for x or y pet.
I look at these things first:
- Availability - What breeds are available for the pet in question.
- Abilities and Combinations - What abilities he can use.
- Opponents and Typing - What is he going to be fighting against.
- Niches and Efficiency - What other pets can do the same thing.
The availability will tell me which breeds a certain pet can have, theoretically pure breeds will almost always perform better than hybrids, not only because they have less raw attributes, but also they generally under perform when compared to their pure breed counterparts.
Then, I look through their abilities to get a feeling of how the pet is designed and how they can be more efficiently used in synchronicity with their spells. Next, I look their typing and what are the possible opponents they will be facing both offensively and defensively.
This whole process gives me a good idea of how I want a certain specie to perform, always respecting other niches that may already exist for determined roles.
For example, let's say I have to choose what breed is the ideal for a random pet: Sifang Otter Pup. The process would be something like this:
Sifang Otter Pup is available in 3 different breeds (S/S, S/B, and B/B).
The stats for each breed are:
SS = H: 1400 P: 260 S: 325
SB = H: 1465 P: 273 S: 289
BB = H: 1481 P: 276 S: 276
For the first slot we have:
Bite: Bite is an average beast move with no secondary effect, therefore more power means stronger hits. If we intend to use Bite, BB is the best breed for this ability.
Gnaw: Gnaw is a move that deals additional beast damage if you strike before your opponent, so striking first speed means bonus damage. Here, SS is the best breed.
For the second slot:
Screech: It reduces the opponent's speed. Most pets at lvl 25 have at least 200 speed, so we are looking at a minimal reduction of 50, so we shouldn't invest in our own attribute if the idea is to reduce the opponent's. Slower breeds make better use of screech, and BB is the slowest one.
Survival: It prevents you from dying the turn you use it and the next one, but it has no priority over other spells and can only be used once every 3 rounds. If you use it before your opponent you will endure two turns with 1 HP, meaning: faster breeds use Survival more effectively.
For the third slot:
Surge: Aquatic weak attack with priority. It will almost always attack first, so there's no reason for having high speed, BB wins here.
Dive: When you use dive you submerge and become semi-invulnerable for 1 turn and hit hard on the next, but can be used only once every 4 rounds. The 1 round of semi-invulnerability forces the opponent to waste one turn by attacking nothing. It may seem like there's no difference which turn he misses since he will miss a turn anyway, but that doesn't help if you are dying before using dive. Attacking first gives you an extra turn, allowing you to hit once more before dying. With this in mind SS is the ideal breed for Dive.
SS appears as "ideal" breed for 3 of his 6 spells, and BB appears for the other 3, so we can assume one of those two must be the "ideal" for this pet.
Now we combine the strategies for each breed...
BB = Bite, Screech, Surge
SS = Gnaw, Survival, Dive
...and observe how each spell interacts with the other:
BB = Screech and Surge have no synergy. Screech reduces the opponent's speed and surge always strikes first, overlapping each other.
SS = The strategy has perfect synergy.
Even though SS already have self-synergy, we can tweak the BB strategy, to see if something more interesting comes up.
1 - BB = Bite, Survival, Surge
This strategy doesn't change much. Survival needs speed to be used more efficiently. It can be used, but it's not ideal.
2 - BB = Bite, Screech, Dive
This one has perfect synergy between the two slots that didn't before. Screech reduces speed and allows you to use dive strategically and also causing slightly more damage than the SS version.
If you dig into it deeper, you will notice that since the opponent will probably be slower after 1 screech Gnaw starts doing more damage than bite. Now, the strategy changes to:
BB = Gnaw, Screech, Dive
Opponents and Typing:
Comparing the two strategies...
BB = Gnaw, Screech, Dive
SS = Gnaw, Survival, Dive
...the kind of opponent you will be the decisive factor to determine which one should be used.
We know that Screech slows down faster opponents, Gnaw is the main source of damage that does higher damage than bite against critters if you are faster, Dive gives you semi-invulnerability and does damage against elementals once every 4 turns. Survival gives you an extra round if you are faster.
BB does slightly higher damage, but needs screech support to perform well. Screech can also be used to help out other members of your team.
SS doesn't rely on the opponent's debuff to perform well and survives a little bit longer than BB, at the cost of doing slightly less damage.
Niches and Efficiency:
Now you have to compare his performance with others pets who have similar use.
Sifang Otter Pup is good to kill critters and OK to counter undead attacks. He also will probably be faster than the opponent most of the time. This is the niche he occupies, and basically what he's good at, so he shouldn't be used in other situations, as other pets would probably out-perform him.
To determine which breed is the ideal, you simply have to weight what strategy fits better whatever it is you intend to do with your team. The decision will probably come from on your own experience or someone else's suggestion.
Personally, here, I prefer S/S over B/B because they don't rely on screech. The 16 points in power BB has over SS is too low to make an impact enough to overshadow the extra turn survival gives. The difference in Health is more than offset by the turn BB waste to set up Screech. In the end, SS will come on top, so we can assume it to be the best possible breed for Sifang Otter Pup.
That's basically it, but you may disagree.
tl;dr: The best breed depends on the set of skills you are using, the opponents you are fighting against, and how you imagine the battle will develop.
What do you do with a pen that only works 90% of the time?