I would not recommend a divider. If you need a divider then you should just take the new fish back. Sevs will get aggressive with other cichlids because it is their nature to be territorial. A divider is always thought to be a temprary fix but it turns out permanent and you still allow the two fish to see each other and stress eachother out.
The only option I can give besides taking the new fish is:
remove all stocking from the tank, do a completel clean with a 50% wc. Rearrange all the decor of the tank. Then re-introduce the fish back into the tank. This will give everyone a fair chance to claim territory and set-up boundaries. If this doesn't work then you need to look into a bigger tank (a much bigger tank than 75G is needed for two adult sevs at 10+ inches anyways) or return the new guy...