For now yes you may be able to DEPENDING on test results and water changes.
However, you do know that the Regal Tang is going to way outgrow that tank don't you. You may be ok for up to a year (if it juvenile now) any longer and will need a much larger tank. The puffer may need more room eventually aswell.
Problem with tangs is that they really swim alot, they will get stressed if they cannot swim and then you end up with white spot.
If you add another fish, you will need to ensure that it does not stress the tang out and you will also need to make sure the tang does not bully the new arrival. To be honest, I would swap the tang for something else that is more suited to your tank size.