The tank is not quite 48" x 18".
I would skip the acei. Demasoni like to be in a big group (15 after removing extra males in a 48x16) so I would do 15 of them and 1m:4f of the labs.
Demasoni are somewhat more difficult to keep than other mbuna so before choosing them, be sure you are determined to do whatever it takes. They are not always a relaxing tank that you add fish to and you enjoy.