You definitely want to get a small clown if you already have a maroon, and keep it in a specimen container until the established maroon no longer tries to kill it. Maroons can be tricky to pair.
If the pet store says it's a gold stripe, you'll have to trust him. And if it turns out to be a white stripe, take it back or at least demand the difference be paid back. I don't think you'll make the boss angry. Besides, you'll only find out if it's not a gold stripe months from now, so you won't be a new customer then. The owner should understand and offer to exchange the fish if it is sold as something it isn't. Keep the receipt if you're really worried.