Unless you want to spend 8-10$ each for laser colours and buy them online, there aren't any bright cories.
Blue gouramis get 4-6", way too big for a 15g.
Honey and dwarf (normal colouration) gouramis are both great.
Please take into consideration that the tetras need a school of 6 and the pandas a group of 4, your stocking idea is, IMHO, too tight and with disproportionate stocking (IE the schooling stuff I said). If you want black phantoms, I would make the stocking:
1 small gourami
6 black phantoms
3 male platies
OR
1 small gourami
6 black phantoms
4 cories
OR
6 black phantoms
3 male platies
4 cories
You might prefer idea #1 since there are virtually no brightly coloured corydoras.