Ok, the first thing to realize is that you have extremely limited space. Saltwater fish are very territorial, and most small fish require copepods and amphipods to graze on during the day. Without live rock, you will not have the benefits of this natural food source. This limits you to an extremely small fish selection. So, rule #1 pertains to the fish. You can only select one of the following fish "combinations".
1) 1 Yellow Tail Damsel or Blue Damsel. No other fish.
2) 1 Ocellaris Clownfish and 1 Falco Hawkfish. Hawkfish
3) 1 Dottyback of the Pseudochromis genus Pseudochromis (Dottybacks)
. No other fish.
4) 3 Kaudern Cardinalfish or 3 Pajami Cardinalfish.
There are no other fish in the hobby which have shown the ability to be successfully kept in small aquariums without live rock, in my opinion. Feel free to throw some fish ideas out there, it is possible I have overlooked some choices and I am happy to comment on possible selections.
So, are you comfortable doing all this work when the fish selections are this limited?