I got 7 fish in my 55. It looks like you only got 6 fish and a shrimp?
I would say a Puffer should be fine, but remember a understocked tank is 100% better than a overstocked tank.
I don't think a puffer would be a problem, just make sure he is not being bullied, or he is not doing the bullying. They like to nip and they will leave small round holes in fins.