Well, even if it's only a 2 inch pleco, a bowl is not suitable. Given the correct conditions, a common pleco will grow very large very quickly, so keeping him in the bowl will only stunt his growth.
As per the original question: there is no way to make your betta less aggressive. You can calm a fish with a darker colored substrate and dark aquarium background, but if the fish is aggressive by nature, there's nothing you can do. Some bettas tolerate tankmates, while others will attack just about anything that moves.