By semi-aggressive, are you wanting to see boisterous behaviour, or have fish that have a predatory side?
Boisterous behaviour: (at least 6) Paraguay Tetras? About the only thing they leave alone are Corydoras hastatus, that look like themsleves on steroids! 3cm fish with a fiesty side. Alternatively, a small group of shell-dwelling Tanganyikan cichlids.
Predatory side: (safely 1, possibly 2) African Butterfly Fish, with either a trio hareem of Gardneri Killifish or (a single) smaller Bushfish species eg. Ornate; Congo.