That's perfectly fine! I have four separate tanks, one is my actual quarantine, one is my FOWLR display, one is my invertebrate display/quarantine, and the last one is my reef tank/grow out tank. I don't think it makes sense to keep such a tiny fish (esp. babies) that may need careful attention and multiple feedings in such a large tank where it may not be able to find/compete for the food. In my experience, the fish either becomes an expensive meal or magically disappears.
I have never had a fish that was less than 2" survive in my 180G (includes Koles, blue tangs, yellow tangs, wrasses, puffers...the list goes on). They get bullied easily, they are out-competed for food, and most of the time they are so shy they starve themselves to death in the hole they hide in--which by the time you notice, it'll already be too late. I prefer to keep smaller fish in a smaller system where I can monitor their health, feed them, and intervene whenever there is an issue like starvation due to stress. Most of my small fish goes into the grow out tank until I have enough confidence that they will make it.
If your fish is only 1.5", I would suggest keeping it where it can have access to food without competition until it grows a bit more. Those tiny babies grows very slowly, but dies very quickly. If it's eating well and growing, do make plans for when it outgrows the tank.
