would 4 albino cories be alright in a 15 g tall planted tank with two small keyholes and one or two shrimp? im not sure what kind they are, theyre about 3 inches and the lfs had them on dark blue gravel, so they looked dark blue
Did a little checking and could not find anything saying they could work together. I don't think I'd chance it, unless someone here can tell you they've had a positive experience with the 2 in the same tank.
A betta and cory cats will be compatible but am not sure about the betta and 2 keyholes. I realize they are a peaceful cichlid and do well in a community tank but I don't think I would chance it in case either the keyholes or the betta became territorial especially in such a small tank. I think you need to do more research about what types of fish the keyholes would be compatible with to see if they will be okay with the cory cats.
I'm sure your original plan would be fine, but I would leave it at that and skip the betta!! The keyholes and corys will not have any problems together. Just make sure to put a couple of hiding spots in the tank, the corys and the shrimp will appreciate it