The groups I play with seem to have a theme leaning tendency. Sometimes that is fun and sometimes it just does not work well at all. One of the more fun games was a party of all elves - sorcerer, druid (healer), druid (summoner), fighter and ranger. The real beauty was that the foes weren't orcs or hobgoblins, but centaurs. Lead by a centaur with several levels of druid, the DM/GM played them perfectly. We never knew that the centaurs were the bad guys until we actually caught them in the middle of a village raid.
So what are a centaur's battle tactics? Well, they are effectively elves, but with a lot more speed, and no ability to climb trees. Give them a few PC levels and they become truly fearsome. A fighter centaur specializing in the lance makes open-field combat deadly for just about anybody who cannot find cover. Centaur rangers do well with bows due to their strength and dexterity bonuses. And not every centaur needs to have character levels, they are decent combatants without levels, so just have the leaders with some levels.
But how do they fight? Well, the book says they are organized into many different sized groups, from solitary to multi-hundred member tribes. How do you want them to fight? As an organized war-band lead by a trained fighter, following planned and executed battle formations - that works. As a chaotic mass of hoof and club, hyped-up on barbarian rage - that works too (be prepared with all the buffed numbers though). What about hit-and-run tactics where the PCs are getting off 1 or 2 long range bow shots after being ambushed - yup, that works.
But these are simple, forest dwelling folks who are usually neutral or neutral good! Yeah, and most elves are chaotic good, how many elven PCs have you seen that didn't fit that descriptor? So, give that centaur a handful of spells and a high charisma and call him a sorcerer - when the PCs see the friendly centaur and come walking out of the forest without their guard up, they either give away all their information to the bad guy or they get a fireball/lightning bolt/flame strike in the face for being *stupid*.
But that will make my PCs want to kill everything or at least not trust anybody! Have you noticed that the goal of most parties is to kick in the front door, take whatever they want and kill anybody who disagrees with them? Occasionally showing the PCs a mirror-monster is a fun way to teach them to keep their guard up and gain a little perspective of what they are doing.