Hero meets the Pet Tamer

Can heros get rid of pets or are they still stuck with the first one they cap?

Yes, a hero can relinquish their pet just as you can. You can find the locations of the pet tamers here.

Last time I checked, only one pet trainer took hero pets, I may be wrong though. That one was in Plains of Jahrir (sp?) I believe.

All pet tamers will take all hero pets. I've swapped pets for my hero's using both the tamer outside Sunspear Great Hall and Ascalon City.

You just have to make sure the hero has Charm Pet skill enabled and the pet active and with them when you talk to the Pet Tamer, you should get the option from the tamer to talk about the hero's pet.