Sure, that's not a bad idea at all. If you are adding all small fish, you can probably add the angels and keyholes all at once, but whichever way you are most comfortable with is fine. For most people, it's easier to find small angels. Quarter size body and larger are typically more hardy than smaller ones. I would also recommend adding some peaceful dithers like flame tetra, lemon tetra, harlequin rasbora, threadfin rainbow, praecox rainbow, stuff like that. You will see more of your keyholes that way. IME with all these species, the more cover and decor you have, the faster they will settle in and color up.