Ok, I've been wondering. I want to get the pet achievement for having only one pet, but I'm really fond of my pets. If I were to abandon them would I be able to re-adopt them, and if so is there a waiting period or can I re-adopt them all back at once?

You can adopt them all back at once but just in case you don't know; Hydrus pets revert back to common in the pound so you'd have to re-potion them and there are chances pound surfers will adopt them while you're abandoning the rest. I'd be best to ask a friend or user if they can hold your pets till you can get them safely back.

Get someone you trust to hold them

I got this Achievement when not many users are online. So you should pick a day like Thuesday or Wednesday and wait until there are less then 300 users online. The chances are pretty high that you'll get them all back. There is a quick way but you need a lot of tabs opened for it XD Open a new Tab for EACH Pet and put the name of the pet in the Search at the Adopt R Us. Open a new Tab for EACH Pet and go to the page where you can abandon it. Click Abandon and then, when you are asked if you really want to, wait. When you have all Tabs open click Yes at the Abandon Tabs as fast as possible. If all Pets but 1 are gone refresh at the 'Pets' page. Then search for your Pets in the Adopt R Us and re-adopt them.

With this way I got all my Pets back safe without any problems :) But there is still the risk that well named Pets are going to be adopted, but you minimize the chances that someone does it when following what I said above xD

Just be sure that if you get a friend to help you, their account is at least one month old :) You have to have a 1 month old account to abandon pets.