Center Piece(s) for 20 Gal. Tall

i personally would not mix cichlids with cories, especially the smaller pygmies. that combo could wind up with a bunch of dead cories, IMO

how about go with one gourami or a pair of dwarf gouramis? your smaller tank size does limit you on what centerpiece fish you can accomodate.

as for the ph, , that will not be an issue. other factors such as kh, gh, tds (total dissolved solids), etc..... those are more of an issue. as long as you acclimate the new fish properly you should have no problem with any of those parameters. and if your new fish come from a local store they most likely have been living in the same local water you have in your tanks at home anyway, so no worries.

Two cichlids that come to mind for your tank would be a mated pair of angelfish. That's about the only thing that will work. You might have to do more water changes because 2 angels are generally kept by themselves in a 20H in most breeders' tanks. The addition of the rasboras and cories may be too much bioload in that tank. Having it planted will certainly help greatly.

You could also consider dwarf cihclids like a trio of apistos or a sexed pair of rams.

Pair of GBRs?

i would say a golden eye would be cool, they get kinda big for that tank but you could try a breeding pair of key holes, or you could go with bolivian rams or apistos