The general rule I go for when making teams is:
*a hard hitter
*a hard counter for what hard counters my hard hitter
*a healer, dodger, staller, something that can survive against a wide variety of pets
I do this by first looking at familes and what does more damage to what or takes more from what. Then I look at skills and look for synergies.
For example, my top three hardest hitters are Lil Rag, Stunted Shardhorn, and Spawn of Onyxia. Now this is going purely by raw attack power not taking moveset into account. There are plenty of other pets who hit hard, like a Fel Flame, monk etc.
If I were to use my Stunted Shardhorn he is a beast, can hit hard, but has no defensive or evasive cd's. He is hard countered by a mechanical and soft countered by a flyer. You want to make sure you have an elemental since they are unique in that they do more damage to and take less damage from mechanicals, the family that does more damage to beasts. Then I would use a crab, turtle, or the Magical Crawdad as the tank/healer/staller. This would leave your team weak against flyers but just make sure not to put an aquatic against the flyer. Or you can use a turtle to make them unswappable and use one of your heavy hitters to bring them down fast. The crawdad can really hold his own most of the time even against most flyers. Pterrordax Hatcling would work too since as a flyer she has an attack that's good against aquatics (which your elemental would be weak against), a heal, and a dodge/damage move.
There are really unlimited combos you can do. Just make sure to take into account the family, their counters, as well as the skills each pet has.