FYI: If you’re returning to WoW after an extended absence prior to patch 5.0.1, it’s possible that you’ll have more than the maximum pet limit and/or more than 3 of a kind. However, this will be the only time that you’re allowed to surpass these limits. You will have to pare down your collection, eliminating your duplicates, in order to add new pets.
Rogues can make fast money from pickpocketing mobs, opening lockboxes and selling items that drop from those on the AH. If you don't have a rogue make one and get him to at least level 16. Rogues can also make some money by picking locks for people and getting tips. Not a great revenue source, but a decent one to supplement multiple strategies for making money. Generally, the usual lockpicking tip is between 50s to 1 gold, the most common being 50 silver. Sometimes, you can get lucky and have someone tip up to 5 gold for lockpicking several (or even just one) boxes. It's always beneficial for a rogue spending time doing repairing, training, etc. in a city to put up a lockpicking advertisement on the trade channel. Just make sure you let the buyers decide the price and that your lockpicking level is high enough.
Although the Battle Pet Trainer will only offer to teach you your race-specific companion, it is possible to add all of these pets to your collection. You can either find them in your faction’s auction house or you can create a character of the race whose pet you would like to learn. Using your new character, you can seek out any battle pet trainer in major cities. After you learn the pet, you can delete the character; the pet you learned will not be removed from your collection.