I think you could easily get away with another small fish....something like a clown or a firefish, some sort of gramma? I wouldn't get anything big, and no messy eaters. JMO.
I've got my 90 gallon stuffed.....yellow tang, dwarf lion, 2 goldstripe maroons, 1 clarkii, sixline wrasse, mandarin, lawnmower blenny, and a southern puffer.
Make sure to keep up on regular maintenance and keep the tank clean...ie. make sure your cleanup crew can handle the extra fish poop, keep a regular schedule for water changes, don't overfeed. I think you'de be fine with another fish. The 1" per 5 gallons of water is simply a rule of thumb. I do take it into consideration. I am slightly over it at 1" per 4.63 gallons