If you had wanted another low swimming schooler, I agree that cardinals would be fun. Also, black neons give some nice contrast. Cories or kuhlis are great. You will see the cories more though.
A few options for different fish are rams, apistos, dwarf gourami, hatchets, bristlenose pleco, harlequin rasboras, smaller rainbow fish (celebes, furcata, threadfin), a single angelfish or mollies.