Now here are your Dragon goddesses.

Also, should I add the Pokemon who can Mega Evolve and gain the Dragon typing to the harem.

Like Mega Charizard X, Mega Sceptile and Mega Ampharos?

Along with them should alternate forms of Pokemon count too?

Like Ultra Necrozma and technically both Arceus and Silvally can become dragon types.
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Latias and Latios:

Rayquaza:

Palkia:

Dialga:

Giratina:

Reshiram:

Zekrom:

Kyurem:

Koraidon:

Miraidon:
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And there we have it, your harem of dragons.