In such a small environment you should never keep more than two clowns and they must
be the same species.
The number and type of fish would greatly depend on their max abult size, behaviour/aggression and if they are active or benthic fish. Trying to keep them >2" would be an asset.
I would suggest looking through >>here
<< and see what strikes your fancy. Put together a wish list keeping in mind the above mentioned criteria and post it so others can help you with an informed descision.