I have a pair of Maroons and have introduced new fish with no bother but then again they have 8ft to get away from each other. My brother also has a Tomatoe Clown which is extremly aggressive towards a hand going into the tank but he has recently a introduced a smaller coral beauty with no problems and thats in a 2ft cube tank.
I think it will probs be ok. I think you would only have a problem with aggression if you were to add more clowns.
I love Marine Bettas! Very envious of you

. Unfortunatly they would snack on all my shrimp if I were to introduce one.
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8' x 2' x 2' - Marine Reef.
6' x 2' x 18" - Wild Caught Zaire 'Moba' Frontosa

4' x 2' x 20" - Malawi
4' x 22" x 18" - Malawi + Victoria
4' x 18" x 18" - Breeding Cuban Cichlids.
Trigon 190 - Male Paratilapia Polleni
20" x 20" x 20" - Marine
30" x 1' x 1' - Hospital Tank
2' x 1' x 1' - Tropical
and
7x - 1' and 2' cichlid fry tanks