Cherry barbs. The males have a really lovely red color and are fairly good schoolers. Most schoolers spread out in smaller tanks, especially if there is not a predator fish to make them feel somewhat insecure and thus school together for protection. For the 15g with 2 cories, I'd go with 6 for now. If you plan on moving them to the 30g also, you could get 6 more and have a nice school. Or you could keep them in the 15g and just get 2-4 more for a school of 8-10.
Also, the cories would really appreciate a few more buddies. They are technically not schoolers but are schoalers and very social. They should be fine as a pair for the time that it takes to upgrade to the 30g, though. I'd recommend at least 6. Depending on what kind you have (and thus the adult size), you might be able to have more.
If you want a really good algae eater, I'd skip the Chinese algae eater. They get way too large for either tank and aren't that great at eating algae anyway. They also stop eating algae as they get older and become very aggressive.
Siamese algae eaters are the ones to look for. Unfortunately, they look a lot like CAEs, and the CAEs are often mislabeled as the Siamese. They would do okay in the 30g but not the 15g.
Nerite snails are a great option. They hardly have any bioload and can be in a tank as small as 2.5g. Plus, there are so many color forms and even spiky ones in several colors and patterns that there is visual interest too. They can be kept with almost any fish, excluding puffers, loaches, and any other snail-eating fish. You could have up to 3 in the 15g or up to 5 in the 30g, more if you feed algae wafers.