I also forgot about Rasboras. A school of 8 Harlequin rasboras would also add some color to the tank.
There are some plecos that won't get too big. You may have to order them online though if you can't get the locally. Some are Bristlenose plecs, Rubbernose or bulldog plecs, and Clown plecos. I'm not sure how the last 2 are at eating algae, but I know bristlenose plecos do a great job. They get about 5 inches. There are also alot of fancy plecos that stay under 6 inches. Most of them are more carnivorous though, so if you are looking for an algae eater, these may not be for you.
A smaller alternative to plecos and the CAE would be otocinclus cats. Your tank seems pretty established, so you could probably exchange the CAE for a group of 5-6 otos when he gets pretty big.
Its deceiving when you buy the CAE small. You wouldn't think they'd be aggressive, or get as big as they do (around 8 inches). I'd definitely think about exchanging him for something more peaceful when he gets around 4 inches or so. You still have some time to find another algae eater type that you like.
or you can get a pair of angelfish
Depends on how nippy the danios are. Some will be ok with angels in the tank, especially if they are in a big group. But I would hesitate to add angels with danios because I like my angels to have pretty long fins...with no shredding.
I've never kept angels with danios but I did buy my angels from a LFS that kept angels and danios together, albeit the tank was smaller than a 55g, and the angels' fins looked horrible.
The angels would also be a perfect target for your CAE. The tend to try to suck the slime coats off other fish, preferably slow moving fish, and it could kill them. I'd atleast remove the CAE before adding any angelfish.
