adding fish

Quick question. So adding fish your suppose to wait about 1 to 2 weeks between each fish correct? I just bought a wrasse today. He's still at the lfs I plan on adding him next friday. That will mark 2 weeks since adding my royal gramma. So here's my question my lfs said that as long has my amonnia has read 0 for a week I should be good till add the wrasse and he sad that this wrasse I got is small and only gets 3 inch it's a Lubbock's fairy wrasse and he said that it wouldn't be much on my bioload since he's small. And my ammonia has read 0 for the past 6 days totomorrow will mark 1 week. Do you think I would be ok to add the wrasse Tuesday. But I'm perfectly fine waiting till next friday. Only reason I wanna add him sooner is that lfs is getting a order in next week and I don't wanna chance the one I bought not getting along with the new batch of fish?