Cool - I love those little Fig 8's! But just fyi - he would do much better in a brackish tank. And if you do that then I believe you may be able to add a bumblebee goby as long as you have a good cave for him.
Don't know for 100% sure that they can go into a 15 though. Double check.
And make sure you've got a really good filter for your little piggy puffer. They make a big mess of their tanks. And of course do your water changes weekly.
Last bit is to be sure you're giving him snails for his beak.
I'm sure you knew a couple of those things but just incase.