A rough guide is around 2.5cm of fish per 9l => 10cm of fish in total. You would want your time to reach this stocking level.
Your occy could grow to around 5cm so that means one other fish. I would probably get another occy as I think they look good together.
Other options are: firefish, flame hawk, bicolour blenny, pyjama wrasse, yellow goby, rainford goby.
Good luck