I personally wouldn't keep an angel in anything smaller than a 30g, which is the minimum tank size requirement on most sites I've visited, including liveaquaria.com which I trust the most. Sure it may be able to survive in smaller tanks, but it sure won't have much room when it's grown out.
I agree that stocking with the intention of getting a bigger tank sometime down the road is a bad idea, especially for a kid who has to go through parents to obtain tank upgrades. If you got a 10g, you should stock accordingly for a 10g. If you get a bigger tank, stock the bigger tank when you get it. You should go by a fish's adult size when stocking, not by how big they are when purchased.