You'll get gifts as you go along in the game, but not all at once since I think it only loads about three gifts at a time. Once you get the potion, go to the basket where you dog is in the Sanctuary and hit X (after selecting it) to change your breed.

There's a Red Setter potion too that comes with new copies of Fable III (along with the Highlander outfit and tattoos). Check your game box for a code insert. 
 
Oh and Boxer is easily the most awesome of the dog breeds, although it's only available with the LCE right now.

Can you switch back and forth?

yep you can switch when ever you wish

Also, your default dog (Collie) will change, at least a little. I've had a couple people drop into my game who's dogs were different, and they were still the default.