For a beginner, I wouldn't recommend anything less than a 4ft 75g for one. For advanced aquarists, such as yourself, 32 bio cube minimum for a single Centropyge. The problem is being mainly benthic feeders, I having enough grazing room, while also providing swimming room. The Atlantic or flame back pygmies would be ok in a mid sized cube no matter the aquarist skill, but they are pretty territorial so I would make them the last addition and only one per 30 gallons.
I'm going to ride the edge with that Reefer, I already have a nox and plan on adding a bicolor in the future. The nox tops out at only 4", and so far seems to be pretty passive and cryptic but is gaining confidence. I'll let it establish once the Reefer is cycled, then add a similar sized bicolor, probably in an acclimation box. The bicolor will outgrow the nox, so I'd like to let it get established and claim territory first.
They will be the only large fish, aside from a pair of clowns. I think she wants a storm and phantom pair, and a black cap basslet are the only fish she's shown an intrest in.