Hate to tell you, but you're overstocked right now for that size aquarium. The Tang and fox face are going to put you over the limit let alone the others. The minimum size for a trigger is 75 gallons and that's pretty much a trigger by itself with maybe a couple small fish.
Go back to the lfs and punch the guy in throat for that kind of advice (not literally, sorry but my dander is getting up). Adding a Trigger (one of the MOST aggressive fish) to that mix is going to lead to stress, disease and death for a lot if not all of your fish.
The niger is one of the least aggressive triggers, but that's like comparing mortar rounds; they both do damage, it's just a question of how much.
The fox is beautfiul but many people consider them aggressive, you've got an interesting mix with agressive fish already so the interactions and stress are going to be curious (to say the least). If the lfs guy was interested in the health of your fish and of maintaining a long term customer he would either:
1. Take the fox back, recognizing that you have too much of a load already on that tank.
or
2. Give you a great deal on a much larger tank and hold your fox until you get things set up.
Suggesting a larger fish to "distract" another from being territorial is ridiculous The reason the fish is being territorial is because it feels cramped already and wants to defend some turf that it can feel comfortable in. Since there's so much encroachment, your fish is getting aggressive to warn everyone to keep away.
Think of it in terms of personal space. If you're talking normally to someone and they maintain a 2-3 foot distance, you feel comfortable. Bring that person 6-12 inches from you and you will continue to take a step back to try and maintain your comfort zone.
I guarantee that if you add a niger, it will eventually kill all of your fish and will outgrow your tank. A trigger is a fast, messy eater and it's size and habits will stress the current tank you have beyond it's capacity.
Your lfs guy is either misinformed and shouldn't be doling out advice until he gets educated, or he was interested in a sale, not the hobby, not your fish and not the obvious joy you get from them. If he was interested in any of those things, he wouldn't have given you that advice.
Let me just step off my soapbox now...
